Variances may be partly due to omissions of important data by the forecaster, partly due to changes in the firm's environment and partly due to factors which can be controlled by the firm. What problems are there in the analysis of variances into these classifications? Are there other classifications which have been omitted from the generalisation? Use the following variances to illustrate your analysis:

(a) wage-rate variance

(b) material-usage variance

(e) fixed-overhead-capacity variance.