Power Calculations in Stata

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Guidelines

Data needed to perform power calculations

You must have:

• Mean and variance for outcome variable for your population
  • Typically can assume mean and SD are the same for treatment and control groups if randomized

• Sample size (assuming you are calculating MDES (δ))
  • If individual randomization, number of people/units (n)
  • If clustered, number of clusters (k), number of units per cluster (m), intracluster correlation (ICC, ρ) and ideally, variation of cluster size

• The following standard conventions
  • Significance level (α) = 0.05
  • Power = 0.80 (i.e. probability of type II error (β) = 0.20

Ideally, you will also have:

• Baseline correlation of outcome with covariates
  • Covariates (individual and/or cluster level) reduce the residual variance of the outcome variable, leading to lower required sample sizes
    • Reducing individual level residual variance is akin to increasing # obs per cluster (bigger effect if ICC low)
    • Reducing cluster level residual variance is akin to increasing # of clusters (bigger effect if ICC and m high)
  • If you have baseline data, this is easy to obtain
    • Including baseline autocorrelation will improve power (keep only time invariant portion of variance)

• Number of follow-up surveys

• Autocorrelation of outcome between FUP rounds

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This article is part of the topic Sampling & Power Calculations

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