The following is from a set of tweets I sent on basic estimating techniques for projects.

The first estmate done for most projects is a Rough Order of Magnitude (ROM) estimate. This usually is done for approval and budget purposes. ROM estimates are usually done on major scope items and are based on past experience or similar other projects. For an auto insurer a project to add a new product to the billing app may have cost $250K. A new product project est. could start at $250K. Once you have a base ROM you can adjust it based on complexity, estimated delivery date etc

The first detailed estimate for a project is done once the initial requirements have been defined.

One trick I use with estimates is to estimate per requirement. This helps avoid scope creep and gold plating because you can see the cost. When estimating tasks get several people to give you an estimate. Use the average and then round up to even increments such as 8, 12, 16 hrs

Use past project metrics to help with estimates. Actual data from past tasks is one of the best measures of how long a task will take.

Next week I will look into more detailed estimating techniques such as function points.