1. price changes in overhead items;

2. inaccurate estimates of one or more item comprising variable overhead;

3. efficiency or inefficiency in the use of the variable overhead

4. inadequate control of an item of overhead cost;

5. volatile behaviour of item of overhead costs; and

6. relationship between variable overheads and direct labor hours is not perfect.

· (Standard variable overhead rate less Actual variable overhead rate ) x actual rate

or

· (Standard variable overhead rate x actual hours) less actual variable overhead.