A normal approximation becomes suitable when the number of trials is sufficiently large and both the expected number of successes and expected number of failures are adequate. These conditions help the binomial count resemble a bell-shaped continuous distribution rather than a strongly asymmetric discrete pattern. Checking both expected counts matters because imbalance can reduce the quality of the estimate.
A continuity correction improves the connection between a discrete binomial count and the continuous curve used by the normal approximation. Because the original outcomes are counted in separate integer values, the correction helps the continuous model represent those count-based probabilities more appropriately. This adjustment supports more useful probability estimates when the normal route replaces direct binomial calculation.
A Poisson approximation is favored when the number of trials is large but the success probability is small. In that setting, the event count is sparse, so a Poisson model can provide a simpler route to probabilities than direct binomial calculation. This differs from the normal route, which depends on sufficiently large expected successes and failures rather than primarily on a small success probability.
An approximation trades some calculation complexity for a model that is easier to work with. The resulting probability is not the direct binomial calculation, but it can preserve useful information about repeated independent trials with two possible outcomes. That tradeoff is especially relevant when exact calculations are cumbersome and the selected normal or Poisson conditions are satisfied.
Start by identifying the number of trials and the success probability, then assess the expected successes and failures. If the trial count is sufficiently large and both expected counts are adequate, use the normal route and include a continuity correction. If trials are numerous but the success probability is small, consider the Poisson route. These checks align the approximation with the data structure.
In statistics, these approximations support probability calculations, confidence intervals, hypothesis tests, and quality-control analyses. They are useful when repeated independent trials produce two possible outcomes and exact binomial calculations become cumbersome. Choosing the appropriate approximation helps analysts simplify the calculation while retaining useful information about the pattern of successes and failures.