Alterations

For a longer test, add an additional check and subtract 1 from the DC (minimum 0).

For a harder challenge, add either 1 to the DC or an extra check. Note that 6 total checks or a modifier of +5 will be very difficult, the PCs only passing about 1/3 of the time. Adding both will result in a harder challenge but not as hard as the next category. A difficult challenge with +5 and 6 checks is very likely to fail. The PCs will pass only 1 out of ever 4 tests.