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.