**[Response: ** Thanks for the catch. That was a pyto. -vlk**]**

Setting 3σ or 5σ thresholds become important when you study the power of the test, defined by one minus the size of type II error, once you reject the null hypothesis, or say signal is significant. Besides many factors, power depends on the sample size. With the same rejection region of the null hypothesis based on 3σ (smaller sample size) or 5σ (larger sample size) thresholds, the power of larger sample is larger than the power of smaller sample; in other words, type II error is smaller with larger sample. Setting high number σ helps to reduce type II error, false negative, or the chance of saying no signal when there is signal. Unfortunately, other factors also determine the power of the test so that a larger σ threshold is not an optimal choice for a reliable source detecting rule, not to mention the cost of collecting large sample and systematic errors.

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