"With varying numbers of drives for each model, it is possible that some bad batches may have made their way into the sample pool, thus further skewing the numb

"With varying numbers of drives for each model, it is possible that some bad batches may have made their way into the sample pool, thus further skewing the numbers." So, to measure drive reliability, the author recommends that you don't count unreliable drives, since it may "skew the numbers"?