Why contamination rate beats contamination count
Losing two containers out of four and two out of twenty are completely different events. Measuring contamination per container is what makes the number comparable.
Most growers track how many batches went bad. That number is almost meaningless on its own, because a batch can be two tubs or twenty.
Rate, not count
Contamination rate is the share of containers affected. Losing two containers out of four is a 50% loss; losing two out of twenty is 10%. The first suggests a process problem, the second suggests ordinary background risk.
Why the timing matters too
Contamination that appears during colonisation usually points upstream, to spawn quality or substrate preparation. Contamination that appears after full colonisation more often points to the environment. Recording the date you noticed it lets you see which pattern you have.
Try it with your own numbers
Mushroom.casa Grow Log records all of this as you go and works out the derived figures for you. Free for three active batches.