Subject to sub-paragraphs (2) to (4), the amount of the earnings to be taken into account for the purpose of calculating N and M shall be calculated or estimated by reference to the average earnings at the relevant week having regard to such evidence as is available in relation to that person’s earnings during such period as appears appropriate to the child support officer beginning not earlier than eight weeks before the relevant week and ending not later than the date of the assessment and for the purpose of that calculation or estimate he may consider evidence of that person’s cumulative earnings during the period beginning with the start of the year of assessment (within the meaning of section 832 of the Income and Corporation Taxes Act 1988) in which the relevant week falls and ending with a date no later than the date of the assessment.

Where a person’s earnings include a bonus or commission which is paid during the period of 52 weeks ending with the relevant week and is paid separately from, or, in relation to a longer period than, the other earnings with which it is paid, the amount of that bonus or commission shall be determined by aggregating such payments received in the 52 weeks ending with the relevant week and dividing by 52.

Subject to sub-paragraph (4), the amount of any earnings of a student shall be determined by aggregating the amount received in the year ending with the relevant week and dividing by 52 or, where the person in question has been a student for less than a year, by aggregating the amount received in the period starting with his becoming a student and ending with the relevant week and dividing by the number of complete weeks in that period.