When performing a study what is a type 1 error?

A Type I error is committed whenever a true null hypothesis is rejected. A Type II error is committed whenever a false null hypothesis is accepted. The best way to explain this is by an example. Suppose a company develops a new drug. The FDA has to decide whether or not the new drug is safe. The null hypothesis here is that the new drug is not safe. A Type I error is committed when a true null hypothesis is rejected, e.g. the FDA concludes that the new drug is safe when it is not. A Type II error occurs whenever a false null hypothesis is accepted, e.g. the drug is declared unsafe, when in fact it is safe. Hope this helps.