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Although breast cancer screening has been shown to work in randomised trials, there is a need to evaluate service screening programmes to ensure that they are delivering the benefit indicated by the trials. We carried out a case-control study to investigate the effect of mammography service screening, in the NHS breast screening programme, on breast cancer mortality in the East Anglian region of the UK. Cases were deaths from breast cancer in women diagnosed between the ages of 50 and 70 years, following the instigation of the East Anglia Breast Screening Programme in 1989. The controls were women (two per case) who had not died of breast cancer, from the same area, matched by date of birth to the cases. Each control was known to be alive at the time of death of her matched case. All women were known to the breast screening programme and were invited, at least once, to be screened. There were 284 cases and 568 controls. The odds ratio (OR) for risk of death from breast cancer in women who attended at least one routine screen compared to those who did not attend was 0.35 (CI: 0.24, 0.50). Adjusting for self-selection bias gave an estimate of the breast cancer mortality reduction associated with invitation to screening of 35% (OR=0.65, 95% CI: 0.48, 0.88). The effect of actually being screened was a 48% breast cancer mortality reduction (OR=0.52, 95% CI: 0.32, 0.84). The results suggest that the National Breast Screening Programme in East Anglia is achieving a reduction in breast cancer deaths, which is at least consistent with the results from the randomised controlled trials of mammographic screening.

Objectives: We undertook this study to calculate the cost per life-year gained in the first round of a screening program for abdominal aortic aneurysm (AAA) and to estimate the costs in a subsequent round., Methods: This was an intervention study, with follow-up for ruptured aneurysms. Men older than 50 years were screened for asymptomatic AAA. Outcome measures included cost per life-year saved and number of men needed to be screened to save one life., Results: The incidence of ruptured AAA was 2.6 per 10,000 person- years in the screening group and 7.1 per 10,000 person-years in the control group. Screening is estimated to have prevented 10.8 ruptured AAA and 8 deaths per year, gaining 51 life-years per year for the study population, and to have reduced the incidence of ruptured AAA by 64% (95% CI, 42%-77%). Each life-year gained during the first screening round cost $1107. To save one life, 1000 men need to be screened and 5 elective operations performed. We predict that a second round of screening can be cost neutral., Conclusions: The cost-effectiveness of screening for AAA compares favorably with screening programs for other disorders in adults.

Background: Opposing views have been published on the importance of ultrasound screening for abdominal aortic aneurysms. The Multicentre Aneurysm Screening Study was designed to assess whether or not such screening is beneficial., Methods: A population-based sample of men (n=67800) aged 65-74 years was enrolled, and each individual randomly allocated to either receive an invitation for an abdominal ultrasound scan (invited group, n=33839) or not (control group, n=33961). Men in whom abdominal aortic aneurysms (> or =3 cm in diameter) were detected were followed-up with repeat ultrasound scans for a mean of 4.1 years. Surgery was considered on specific criteria (diameter > or =5.5 cm, expansion > or =1 cm per year, symptoms). Mortality data were obtained from the Office of National Statistics, and an intention-to-treat analysis was based on cause of death. Quality of life was assessed with four standardised scales. The primary outcome measure was mortality related to abdominal aortic aneurysm., Findings: 27147 of 33839 (80%) men in the invited group accepted the invitation to screening, and 1333 aneurysms were detected. There were 65 aneurysm-related deaths (absolute risk 0.19%) in the invited group, and 113 (0.33%) in the control group (risk reduction 42%, 95% CI 22-58; p=0.0002), with a 53% reduction (95% CI 30-64) in those who attended screening. 30-day mortality was 6% (24 of 414) after elective surgery for an aneurysm, and 37% (30 of 81) after emergency surgery., Interpretation: Our results provide reliable evidence of benefit from screening for abdominal aortic aneurysms.

Background: The study was an update at 10 years of a randomized trial of the efficacy of screening for abdominal aortic aneurysm (AAA). The extent of benefit, feasibility and compliance were examined, and reasons why this intervention may fail a proportion of those screened were identified., Methods: A total of 6058 men aged 65 years and over were randomized to a group invited to attend ultrasonographic screening or to a control group. The mortality rate from AAA in the two arms of the trial was compared using a Poisson model. Analyses were by intention to treat., Results: There was a 21 per cent reduction in mortality rate from AAA over the 10-year follow-up (relative risk 0.79 (95 per cent confidence interval 0.53 to 1.40)). The observed relative mortality reduction peaked at 4 years with a 52 per cent reduction in the study group. Eighteen of 24 AAA deaths in the study group were among those who did not attend the first screen, or failed to comply with the follow-up protocol., Conclusion: A greater awareness of the benefits of full participation in a screening programme could provide a larger and sustained mortality reduction.

Objectives: To assess the accuracy of screening for abdominal aortic aneurysms (AAAs) by ultrasound (US)., Setting: An aneurysm screening programme in Huntingdon., Methods: False negative tests were identified by tracing all patients with a ruptured aneurysm who were screened and then finding the number classified as normal on US. False positive tests were identified by calculating the number of aneurysmal aortas on US that were classified as normal on CT. Measurement variability of the infrarenal aortic diameter between US and CT was estimated., Results: 14 out of 93 patients with a ruptured AAA since 1991 had been screened. No ruptured aneurysm had been classified as normal on US. All 64 patients with an AAA larger than 4.5 cm on US had their aneurysm confirmed on CT. The mean difference between CT and US measurements was 4 mm. The limit of variability between CT and US was 12 mm., Conclusion: No false negative scans were found using a cut off point of 3 cm as abnormal. No false positives were found if subjects with an AAA exceeding 4.5 cm were referred for further procedures. A serial US screening policy has excellent screening performance, justifying its use as a screening tool.

Aim: To investigate the efficacy of a single ultrasonic scan at age 65 to identify patients at risk from ruptured abdominal aortic aneurysm (AAA)., Method: A total of 6058 men aged 64-81 were recruited to a randomised trial, and 3000 were invited to attend a single screening test. An additional population of 1011 men was offered screening as they reached age 65. If a normal aorta was identified in this sub-group, further scans were offered at two-yearly intervals. Follow up and treatment of those identified as having an aortic dilatation of 3 cm or greater was undertaken. All subject groups were monitored for deaths occurring over the study period, and date and cause of death were recorded., Results: A total of 2212 men attended screening in the randomised trial; the overall compliance was 74%, and prevalence of AAA was 7.7%. Compliance decreased, and prevalence increased, with age. Mortality from ruptured AAA was reduced by 68% at 5 years (screened group compared to the age-matched control population), and by 42% in the study arm (screened and refusers) compared with controls. The benefit persisted at ten years (53% and 21% respectively). Of the uncontrolled sample of 1011 men offered a scan at age 65, 681 attended and 649 of these were found to have a normal aortic diameter; re-screening demonstrated new aneurysm development in 4% over ten years. The aortic diameters of the new AAAs were under 4 cm and would therefore have a low risk of rupture.1 Mortality from rupture in all those with an initially normal aortic diameter was low, at 1 case per 1000 scans over ten years., Conclusion: Screening once for AAA at age 65 can identify the majority of AAA that are of clinical significance and can identify a large population at low risk from rupture who do not require surveillance. This policy has been effective when combined with selective treatment in reducing the risk of rupture for ten years in those who attend the screening programme., (Copyright 2001 Harcourt Publishers Limited.)

Background: Preliminary retrospective data suggest it is possible to identify impalpable breast cancer in women presenting with a family history of breast cancer under the age of 50, by using regular mammography. In consequence, this service is offered in a number of centres in the UK. The effectiveness of such a service, however, has not been fully evaluated., Methods: We propose to perform such an evaluation in a cohort of 20000 women under the age of 50 with a significant family history of breast cancer, given regular mammographic surveillance over 5 years. Comparison of surgical and pathological data with completed and ongoing population screening trials using analysis techniques of varying complexity will be performed to obtain an accurate prediction of future breast-cancer mortality reduction. The formal aims are: i) to estimate the difference in breast-cancer mortality in women under the age of 50 with a significant family history of breast cancer having regular mammography, compared with those not being screened; ii) to estimate the cost-effectiveness of regular mammography in this group of women, compared with no screening. The increase in health service resource use attributable to such a policy will be compared with no screening, and costed. Incremental cost-effectiveness ratios of implementing the standardised mammography strategy compared with no screening will be presented in terms of the additional cost per cancer detected, per life saved and per life-year saved.

Objective: To perform a pilot study to compare ultrasound and mammographic screening with particular respect to specificity, in a cohort of women at moderate family history risk of breast cancer., Material and Methods: High resolution ultrasound of both breasts was undertaken in 149 women at moderate risk of breast cancer who were referred for mammographic screening. The two tests were read blind to the other. Core biopsy was undertaken for solid lesions found on ultrasound., Results: 149 Patients, mean age 42.15 years, were screened with bilateral ultrasound. 90 Examinations were normal. 46 Women had cysts, two had lesions previously biopsied and found to be benign, and one had a lymph node. 10 Focal solid lesions were biopsied on account of their ultrasound findings, 6.7% (95% confidence intervals (95% CIs) 2.7% to 10.7%). All but one of the 149 mammograms were normal. In this case both mammography and ultrasound showed a focal solid lesion that at core biopsy was found to be a fibroadenoma. Histology showed seven fibroadenomata, two areas of fibrocystic change, and one adenoid cystic carcinoma. Positive predictive value for biopsy was 10%. Mean follow up was 13.7 months. There was one interval cancer., Conclusion: Screening with breast ultrasound, in a cohort of women at moderate risk of breast cancer due to family history, has an acceptable biopsy rate. Screening with ultrasound and mammography in patients with an increased risk of breast cancer may be beneficial and a randomised study to examine issues of acceptability, reproducibility, and cost effectiveness is apt.

Objectives: To determine the incidence of asymptomatic abdominal aortic aneurysms and the implications for an ultrasound screening programme in England and Wales., Methods: First screen data were obtained from the Chichester and Huntingdon screening studies and used to estimate the prevalence of abdominal aortic aneurysms. The incidence of new, asymptomatic aneurysms was estimated from the prevalence rates observed in the Huntingdon screening study., Setting: Screening programmes in Huntingdon and Chichester using ultrasound to screen all men over the age of 50 and men over age 65 respectively., Results: The prevalence of abdominal aortic aneurysms ranged between 5.32% and 8.02% and between 6.18% and 9.88% of men aged between 65 and 79 in Chichester and Huntingdon respectively. Annual incidence rates, estimated by age, rose steadily reaching a peak of 0.67% of the Huntingdon population per year at age 65. Thereafter incidence falls. Estimates of the incidence of new asymptomatic abdominal aortic aneurysms, based on the observed prevalence data, were calculated and showed a peak at age 65., Conclusions: Hypotheses are offered to explain this unexpected early peak in incidence. This information should allow the definition of the optimum age for screening, and the relative benefits of screening at different intervals if widespread screening is adopted in the future.

Objective: To study the trends in the incidence of invasive cervical cancer in East Anglia., Design: Statistical analysis of age specific incidence rate for the period 1971-93 using East Anglian Cancer Registry data., Subjects: All cases of invasive cervical cancer registered with the East Anglian Cancer registry, diagnosed in the period 1971-93., Main Outcome Measures: Changing incidence of cervical cancer., Results: For the 20 years 1971-90, trends varied widely by district and by age group, with little discernible overall effect of the increasing screening activity. Since 1990, rates have fallen sharply in the age groups targeted for screening, with a reduction of 34% (95% confidence interval 26% to 42%) from that expected based on 1971-90 trends. This fall was preceded by a rapid rise in the national uptake of screening. A shift to more favourable stage at diagnosis has also occurred., Conclusion: Changes in the organisation and management of the national screening programme introduced in 1988 and 1989 seem to have led to substantial improvements in effectiveness.

Close monitoring of data from individual programmes is required to evaluate the potential of the breast screening programme to reach the Health of the Nation target of 25% reduction in breast cancer mortality in the invited age group by the year 2000. This paper outlines the use of indirect age standardisation techniques to compare the performance of individual programmes in terms of their invasive cancer detection rates. Expected invasive cancer detection rates are calculated by applying data from the Swedish two county study to estimated England and Wales background incidence rates for different age strata. If the national programme overall meets these targets then the required mortality reduction should be achieved. The same method can be used by other (national) screening programmes by applying the relevant background incidence figures to produce internationally comparable data.

The sojourn time, time spent in the preclinical detectable phase (PCDP) for chronic diseases, for example, breast cancer, plays an important role in the design and assessment of screening programmes. Traditional methods to estimate it usually assume a uniform incidence rate of preclinical disease from a randomized control group or historical data. In this paper, a two-parameter Markov chain model is proposed and developed to explicitly estimate the preclinical incidence rate (lambda 1) and the rate of transition from preclinical to clinical state (lambda 2, equivalent to the inverse of mean sojourn time) without using control data. A new estimate of sensitivity is proposed, based on the estimated parameters of the Markov process. When this method is applied to the data from the Swedish two-county study of breast cancer screening in the age group 70-74, the estimate of MST is 2.3 with 95 per cent CI ranging from 2.1 to 2.5, which is close to the result based on the traditional method but the 95 per cent CI is narrower using the Markov model. The reason for the greater precision of the latter is the fuller use of all temporal data, since the continuous exact times to events are used in our method instead of grouping them as in the traditional method. Ongoing and future researches should extend this model to include, for example, the tumour size, nodal status and malignancy grade, along with methods of simultaneously estimating sensitivity and the transition rates in the Markov process.

Study Objective: The aim was to demonstrate the benefits of breast cancer screening on mortality., Design: The study was an evaluation of a breast cancer screening programme by means of different approaches: (1) a case-control study, breast cancer deaths being the cases; (2) comparing the numbers of breast cancer deaths in screened and unscreened women; (3) comparing breast cancer mortality before and after start of the programme; (4) comparing breast cancer mortality in different large cities; (5) comparing screening activity with mortality reduction., Setting: The setting was a breast cancer screening programme in the city of Utrecht, the DOM project, for women aged 50-64 years old at intake, birth cohort 1911-1925. The programme started in 1974, and there were five screening rounds up to 1984. Participation rate in the first round was 72% (14,697 women)., Main Results: (1) Screening was protective against dying from breast cancer, odds ratio 0.52, with a stronger effect in older women and no evidence of confounding; (2) risk ratio of dying from breast cancer for women in the response group was the same as the odds ratio, 0.52; (3) breast cancer death rate after the start of the project was nearly 20% lower than before the project started; after correcting for women who could not have benefited from screening the reduction was 33%; (4) a rise in breast cancer mortality in birth cohort 1911-1925 seen in other large cities without a screening programme due to aging of the cohort was not seen in the city of Utrecht; (5) mortality reduction followed the screening activity with a time lag of approximately 5 years., Conclusions: Early diagnosis of breast cancer by mammography reduces breast cancer mortality in women 50-64 years old at intake; different approaches to the evaluation of the project give different estimates of the screening effect, making clear that the effect depends on the intensity of the programme.

The results of the Swedish two-county trial of mammographic screening for breast cancer are presented, updated to December 31, 1990, which is an average of 10.8 years follow-up per person. The main result of the trial in terms of breast cancer mortality remained the same: compared with the control group, the group invited to screening had a relative breast cancer mortality of 0.70 (P = 0.0002) with 95% confidence interval (0.58, 0.85). Analysis of survival showed that relative to the control group, the cancers detected at prevalence screen, incidence screens, and in the interval between screens had a good prognosis, whereas cancers detected in those who had refused screening had a very poor prognosis. When adjusted for tumor size, lymph node status, and tumor grade (differentiation), the better survival associated with incidence screen and interval detection was largely accounted for, indicating that the benefit of incidence screening is largely achieved through the effect of screening on the three prognostic variables, notably size of the tumor. Results indicate that to achieve a substantial mortality reduction, 50% of screen-detected invasive cancers should be less than 15 mm in diameter, at least 30% of screen-detected grade 3 tumors should be less than 15 mm, and at least 70% of screen-detected tumors should not have lymph node metastases. The percentage of grade 3 tumors of a given size should be the same in screen-detected cancers as in clinically detected, and breast cancer prevalence at first screen should be at least three times the expected incidence rate in the absence of screening. This should be achieved without the recall rate for further examination exceeding 9%, and procedures including further imaging techniques and fine needle aspiration or core biopsy should be used before resorting to surgical biopsy. These aims can be achieved in specialist screening centers if particular attention is paid to resources for screening and diagnostic evaluation, specialist training of clinical and technologic screening staff, and ongoing monitoring of mammographic quality, recall rates, and the attributes of the tumors detected.

Randomised trials of screening have clearly demonstrated a substantial reduction of the order of 40% in breast cancer mortality among women aged over 50 years when regularly screened every two to three years by mammography. No other screening modality has been demonstrated to be of benefit, nor has screening of younger women. The major problem of breast screening at present is the need for greater understanding of the heterogenous natural history of the disease. The extent to which earlier diagnosis improves prognosis is poorly understood, but crucial to determining the full potential of screening.

The evaluation of screening programmes for cancer is discussed with particular reference to lead time and length bias. The epidemiological approach, concentrating on the population screened instead of the cases detected, leads to a useful evaluation of the concepts of lead time and length bias, being closely related to the natural history of the disease in question. This is shown by reviewing screening programmes for these different diseases: breast cancer, lung cancer and cervical cancer.

It is important that the introduction of breast screening is closely monitored. The anticipated effect on breast cancer mortality will take 10 years or more fully to emerge, and will only occur if a succession of more short-term end points are met. Data from the Swedish two-county randomised trial provide targets that should be achieved, following a logical progression of compliance with the initial invitation, prevalence and stage distribution at the prevalence screen, the rate of interval cancers after the initial screen, the pick-up rate and stage distribution at later screening tests, the rate of interval cancers after later tests, the absolute rate of advanced cancer and finally the breast cancer mortality rate. For evaluation purposes, historical data on stage at diagnosis is desirable; it is suggested that tumour size is probably the most relevant variable available in most cases.

Time trends in mortality from cervical cancer in Denmark, Finland, Iceland, Norway, and Sweden since the early 1950s were investigated in relation to the extent and intensity of organised screening programmes in these countries. In all five countries the cumulative mortality rates (0-74 years) fell between 1965 and 1982. In Iceland, where the nationwide programme has the widest target age range, the fall in mortality was greatest (80%). Finland and Sweden have nationwide programmes also; the mortality fell by 50% and 34%, respectively. In Denmark, where about 40% of the population are covered by organised programmes, the overall mortality fell by 25%, but in Norway, with only 5% of the population covered by organised screening, the mortality fell by only 10%. The results support the conclusion that organised screening programmes have had a major impact on the reduction in mortality from cervical cancer in the Nordic countries.

The evaluation of screening programs for cancer is considered. Initial evaluation has to be in terms of mortality, but increasing importance should be attached to measures which evaluate the interaction of early detection with the disease process. These include the degree to which diagnosis is advanced, both in time and in stage of disease, and the ability of the screening test to identify presymptomatic lesions. Randomized trials are of major importance in establishing--in quantitative terms--the benefits of screening; in their absence, case-control methods can be adapted for this purpose, but cannot provide evidence of the same rigor. Currently, only screening for breast cancer and cervical cancer have been fully demonstrated to be effective.

A workshop of the Project on Evaluation of Screening Programmes of the International Union against Cancer (UICC) was held in Lyon, France. on November 20-22, 1984. The focus of the workshop was on screening for gynaecological cancer, especially for cancer of the cervix uteri. This report summarizes the presentations, conclusions and recommendations from the workshop.

Age-specific incidence curves for clinical cancer of the cervix in England and Wales show progressive changes over the period 1963-1978; in particular, a large reduction in incidence is seen in the age group 35-54. Since screening on any scale began in the early 1960s, we have investigated how much of this reduction in incidence in the middle age range can be attributed to detection of pre-invasive disease. Data on registrations of in-situ cancer have been used to estimate the patterns that might have been observed in the absence of screening. The results indicate clear cohort effects on incidence, with rising rates in the generations born 1906-1921 and since 1931, with a fall in the decade between. In addition to this, screening has probably led to a substantial reduction in the number of cases of clinical cancer in women aged 35-54, but has had little effect over the age of 60 where virtually no screening has been performed. Below age 35 the observed increase in incidence may be considerably less than it would have been in the absence of screening.

The Swedish two county trial of breast cancer screening is now in its tenth year. This paper presents detailed results on mortality from breast cancer and from all other causes, and on the population denominators at risk for each of the first 8 years of follow up, for each county separately. These data represent a two year update on the last major report. Results show an increasingly significant deficit in deaths from breast cancer among the 77,092 women invited to screening relative to the 56,000 not invited (RR = 0.68, p = 0.002), with no significant difference between the effects of screening in the two counties (p = 0.5). These results remain the same when adjusted for age. Analysis of all cause deaths shows no significant effect of screening (p = 0.5), nor was there any significant effect of screening on deaths from all causes other than breast cancer (p = 0.9). The rates of deaths from intercurrent illness in breast cancer cases were almost identical in the group invited to screening and the group not invited (p = 0.7). This result remained the same when adjusted for age. We calculate that in the age group 50-69 at entry, one breast cancer death was prevented per 4000 woman/years, per 1460 mammographic examinations, per 13.5 biopsies, and per 7.4 breast cancers detected.

A commonly used estimate of a screening test's sensitivity, based on the prevalence at screening and the incidence of interval cancers, is shown to be logically unsound and quantitatively poor. An alternative simple estimate is proposed, based only on incidence rates.

The case-control approach has been proposed for the evaluation of the efficacy of screening for cancer. It provides information at the level of the individual subject and allows direct estimation of the relative risk of not being screened. The criteria for the choice of the case and control series are examined, with reference to the type of cancer considered and the precise goal assigned to the screening programme. The definition of the exposure is discussed, as well as criteria for the avoidance of commonly occurring biases. Typical situations where case-control studies of screening efficacy appear to be the method of choice are presented.

In this paper, the authors show how data on the observed prevalence of disease at a screen and on the incidence of disease during intervals between screens may be used to estimate jointly the distribution of the length of time during which individuals remain in the pre-clinical state and the sensitivity of the screen. Apart from being of biologic interest, such estimates may be used to evaluate the length of time by which the date of diagnosis could be advanced by screening (the lead time) as well as to predict the relative effectiveness of various alternative screening strategies. The methodology uses only information which should be routinely available in the course of a typical screening program, and makes only rather mild statistical assumptions. The authors illustrate the methods with breast cancer screening data from the Health Insurance Plan of Greater New York (HIP). Although these data have been analyzed by several other authors, the present approach is the first which simultaneously gives estimates of the pre-clinical state duration, the sensitivity of the screening method, and the underlying incidence rate in the screened group, while also taking into account the problem of length-biased sampling.

Mass screening programs for cancer of the cervix were introduced at different times and to different extents in the Scandinavian countries. Iceland and Finland introduced nationwide screening programs in the early 1960s. Ten years after their introduction, marked reductions in incidence and mortality of invasive cervical cancer were observed, seen specifically in the age groups to which screening was targeted. In Sweden and Denmark, mass screening was introduced on a county basis, and changes in incidence and death rates from cervical cancer were recorded which reflected the degree to which organized mass screening had been introduced. These data demonstrate that mass screening scheduled every two to five years can reduce the incidence of invasive cervical cancer by 80%. No evidence is available from Scandinavia that more frequent screening reduces the risk further.

The results of a cervical cancer screening program in a population of 22,106 women aged over 25 years in a rural county of China are evaluated in a case-control analysis. The relative risk of cervix cancer was 0.33 in women with three or more negative tests (compared with one or less) and 11.4 for women whose last negative smear was 8 or more years previously (compared with 2 years or less). Attendance at the screening program was positively associated with some risk factors for cervix cancer (sexual behavior of the woman and her partner) and negatively associated with others (genital hygiene). Controlling for the confounding effect of these risk factors resulted in only small changes in the estimate of the protective effect of screening.

To estimate the relative risk of invasive cervical cancer in each succeeding year after a negative screening result the screening records of all women tested in the north east of Scotland were examined as the basis for a case-control study. The cases consisted of 115 women in whom invasive cervical cancer had been diagnosed in 1968-82 and who had appeared in the screening records at least once before diagnosis. For each patient five controls were selected from women of the same age who appeared in the screening records before the date of diagnosis in the patient. If the patient's cancer had been detected by screening the controls were chosen from women of the same age screened the same year. A comparison was made between cases and controls of the number of negative smears taken before the diagnosis. The results showed a high relative protection (inverse of the relative risk) in the first two years after a negative test, falling steadily as time since the last negative test elapsed. Even after 10 years, however, a considerable residual effect was observed.

The performance of a screening programme depends on a number of parameters, which may need different types of information for their estimation. In this paper, two characteristics of interest, the sojourn-time distribution and the sensitivity, are considered, and estimation procedures, based on data which should be available from a mass screening programme, are proposed. The methods are applied to published data from the HIP breast cancer screening project, to which a negative exponential distribution for the sojourn-time distribution fitted better than the other families of distribution considered.