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Purpose: The spatiotemporal index (STI) is a widely used approach for measuring speech pattern stability across multiple repetitions of a stimulus. In this study, we examine how methodological choices in the implementation of the STI (including the number of repetitions, length of stimuli, and parsing procedure) can affect its value. Method: To evaluate how each methodological decision affects the STI, we use a synthetic data framework that allows for the generation of random productions of the template phrase 'Buy Bobby a Puppy' at different stability levels. Within this framework, we conduct three experiments: Experiment 1 investigates the effects of the number of repetitions, Experiment 2 investigates the effects of stimulus length, and Experiment 3 investigates the effects of parsing errors. Results: In Experiment 1, we observed that STI values based on fewer repetitions will systematically underestimate larger repetition estimates. Experiment 2 showed that STI values will tend to be higher when calculated on longer (multimovement) stimuli independent of any differences in the stability of the underlying speech patterns. Finally, in Experiment 3, we showed that even minor parsing errors ([approximately equal to] 10 ms) increase the value of the STI. Conclusions: The results of this study illustrate that even minor choices in the implementation of the STI can have a noticeable impact on the resulting value. These findings highlight the care that needs to be taken when designing studies and comparing STI values across studies to ensure that different STI values are capturing real differences in motion pattern stability rather than trivial methodological variation., The spatiotemporal index (STI) is a measure of movement pattern stability exhibited over repeated performance of the same motor task (such as speech production). By incorporating repeated utterances across the [...]