Performance metrics such as mean-bit-error rate and probability of fade for free-space optical communication (FSOC) applications using intensity-modulation direct detection are theoretically calculated based on probability density functions (PDFs) describing irradiance fluctuations. Theoretical calculations using common PDF models can result in significant errors in prediction of performance metrics. In particular, these models do not consider the change in skewness of the distribution as the aperture size increases, and often positively skewed distributions (right tailed) are used to model scenarios where the true statistics are negatively skewed (left tailed). We evaluate the magnitude of errors in the prediction of bit-error rate and probability of fade based on simulation data for a collimated Gaussian beam in a realistic FSOC scenario for two strengths of turbulence and varying aperture sizes. The PDF models considered are lognormal, gamma–gamma, inverse-Gaussian-gamma, fractional exponential, exponentiated-Weibull, 3-parameter-Weibull, and normal distributions.