During the last 6 weeks, the actual costs of materials for Brennen Company were as follows:

Week 1.............$57,600.............Week 4.............$68,400

Week 2.............$60,000.............Week 5.............$69,000

Week 3.............$63,000.............Week 6.............$69,500

The standard materials cost for each week was $60,000 with an allowable deviation of ±6,000.

Required:

Plot the actual costs over time against the upper and lower limits. Comment on whether or not there is a need to investigate any of the variances.