Background: Sample size estimations are completed to determine the number of participants needed to highlight a relevant treatment effect [1]. This is crucial to ensure that the results are reliable. Estimating sample size can also assist researchers by informing the financial outlay for a project and minimising any over spend. Purpose: In this study, different methods to estimate sample size using PFO data were utilised. PFO data collected using the test re-test method was employed to estimate sample size. PFO results from previous research were also applied to estimate sample size [2,3,5,6]. Methods: Subsequently, a group of endurance-trained women (n=5) aged 36 (+/- 2.4 years) were recruited from the main study. Menstrual cycle (MC) length was established and within the MC, the mid-follicular phase (MF) (low hormone phase) was determined [4]. Identifying the MF phase of the MC would allow for FATMAX test re-test`s to be conducted when sex hormones were least likely to change or impose an effect on substrate utilisation. Tests were separated by 24 hours to allow participants to recover, to ensure hormonal change was minimal and to test the reliability of the results collected. Results: Results from previous research were statistically analysed individually and in group form to estimate sample size. When analysing PFO data from previous research, a mean sample estimation of 7 participants was suggested. Test re-test PFO results were assessed for reliability (ICC=0.82) before sample size estimations were conducted. Individual analysis from the test re-test method outlined a mean sample estimation of 14 participants, and group analysis from this method suggested a mean sample estimation of 14 participants. An overall mean sample from all estimations outlined a sample size of 12 participants.
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