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HMRC internal manual

National Insurance Manual

Earnings Periods: Employee earns a fee for each session they work: Payment made irregularly and no regularity found

If an employer pays an employee a fee at irregular intervals for each session they workand neither NIM08200 nor NIM08210 apply,the earnings period is the interval between the payments or a week, if that is longer.

See NIM09910 if the employee is an irregular harvest worker.