Just as a car mechanic has dozens of specialized tools to draw upon --- a professional economist has a tool kit that he or she frequently draws from to analyze economic problems.

If I want to determine how dry weather will affect corn prices -- I will use supply and demand models along with price elasticities.If I want to determine whether we should build a dam north of Ames for flood control -- I will use benefit/Cost analysis.

Different economists using the same tools can arrive at very different conclusions. -- Or appear to do so.

Example: The conclusions reached using benefit/cost analysis depend partly on the interest rate used. If a high interest rate is used, the costs are likely to exceed the benefits. The appropriate interest rate to use can depend on how the project is to be financed.