Which of the following is not a method used to determine an employer's unemployment insurance experience rating?

The method that is not used to determine an employer's unemployment insurance experience rating is the payroll ratio. This is because the experience rating generally assesses how much an employer has contributed to the unemployment insurance fund relative to the claims made by former employees. The benefit ratio, benefit-wage ratio, and reserve ratio are all calculations that directly relate to the employer's experience with unemployment claims, reflecting how the employer's workforce has interacted with the unemployment system.

The benefit ratio looks at the number of unemployment benefits paid out to former employees compared to the total wages paid by the employer, effectively measuring the employer's experience with unemployment claims. The benefit-wage ratio considers the relationship between the total benefits claimed and the total wages, using this to determine if an employer is a higher or lower risk for unemployment claims. The reserve ratio provides insight into the balance of funds available in an employer’s unemployment insurance account, comparing the unspent balance to the unemployment claims, which also plays a role in evaluating the employer's experience rating.

In contrast, the payroll ratio does not provide a relevant measure for calculating experience ratings, as it typically involves overall payroll figures rather than focusing on claims or benefits specific to the unemployment insurance system. This is why it is the correct answer in this context.