Chapter 6 deals with procedure for establishing the facts upon which sentences will be based. Accurate fact-finding is essential to ensure that an appropriate sentence is imposed and that disparities are avoided. However, workability and administrative efficiency also are important to the proper application of the sentencing guidelines. Consequently, the Commission chose to focus on a relatively manageable number of frequently occurring factors and to avoid a highly detailed, all-encompassing weighting system that considers every conceivable nuance. The presentence report, together with requirements for advance statements of the position of the parties and stipulations as to relevant sentencing factors, provide mechanisms for ensuring accuracy and fairness and facilitating dispute resolution. Finally, the guidelines also provide for more formal mechanisms in resolving disputes and contested factual issues. Because some 85 percent of Federal criminal convictions result from guilty pleas, the Commission examined empirical data on the effect of guilty pleas. Analyses indicate that the average time served is 30 to 40 percent lower when conviction is on a guilty plea than when the conviction is obtained through trial.