In addition to variance analysis and earned value analysis, another tool for project control is the Critical Ratio analysis. The critical ratio CR= CPIxSPI.
The ideal CR will be 1.0 Ratios greater than 1.0 imply project doing well on both cost and schedule while ratios below 1.0 imply poor performance.
A useful guide might be the use of a chart to show the CR over time and when intervention (control) might be needed. An example is attached critical-ratio-graph
The graph shows in one example when ratios above and below 1.0 might be ignored and when project management action is needed.