The effective failure rate, λE(τ), is the unconditional expected number of
failure per time unit as a function of the maintenance interval τ.
As a introduction consider an item with an increasing failure rate function, z(t):
If no preventive maintenance is conducted (the left part of the figure)
and the item is only maintained upon a failure, the effective failure rate will be quite high.
If we maintain at time τ1 (the middle part of the figure), we may "remove"
some part of the right hand part of the bathtub curve, resulting in a lower effective failure rate.
If we further reduce the maintenance interval, say to τ2 (the right part of the figure),
the resulting effective failure rate could be quite low.