when you are combining results for wassce and novdec, the best grade for each subject is used in the final aggregate tabulation. For instance, if you had B3 in maths for wassce and C5 in novdec, the B3 will be used in the final calculation. consider the example below.
WASSCE : maths B2, english C4, social E8, science F9
NOVDEC: maths C5, english B2, social C4, science C5
FINAL RESULTS USED
maths B2, english B2, social C4, science C5