How to measure prevalence of patient quality of life and response rate to disease modifying treatment (they are more than one) to use this prevalence in measuring sample size??
If you have different prevalence and response rates from different sources you can estimates sample size separately using these rates and use maximum sample size for the study OR you can select baseline estimates from most relevant or recent source to calculate sample size.
For cross-sectional surveys if the prevalence is close to 50% it gives maximum sample size using simple asymptotic formula .
I too suggest when prevalence is not clearly known or varying prevalence of different factors, 0.5 prevalence is a better idea for sample size calculation. However to ensure good sample size you would decide on taking design effect as 1.5 or 2 even depending on resources.
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