As many stated, weather, missing member due to sickness, vacation, or personal problems, resource problems, and budget problems can cause affect schedule variance. All these issues can cause work delay. These can be unpredictable but the risk management should help assist in the problem. The project manager should be prepared for all issues and the should be able to adjust the schedule so the issue is fixed. The risk management should serve to build a backup plan if there is any schedule delays due to the ones I named in the beginning. They could main solution would be calling for recuitment to improve and fix the work delay.