In reports on ophthalmological research the results of measurements on the eye are often expressed as mean and standard deviation based on m patients, n eyes (n greater than m). This approach leads to t tests that are invalid because the measurements on the two eyes of one subject are usually related, not independent. In a simulation study involving intraocular pressure data analysed in this way, the null hypothesis of no difference between groups was rejected at a nominal alpha = 0.05 level in 39 out of 200 simulations; thus the true alpha was nearly 0.2. This approach is excessively prone to produce false positive results.
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