When a program is ready to be tested, programmers run it to ensure that the instructions are correct and will produce the desired information. They prepare sample data that test every part of the program and, after trial runs, review the results to see if any errors were made. If errors do occur, the programmer must make the appropriate change and recheck the program until it produces the correct results. Finally, programmers working in a mainframe environment prepare instructions for the computer operator who will run the program. They may also contribute to a user's manual for the program.
Programs vary depending upon the type of information to be accessed or generated.
For example, the instructions involved in updating financial records are different from those required to duplicate conditions onboard an aircraft for pilots training in a flight simulator. Although simple programs can be written in a few hours, programs that use complex mathematical formulas or many data files may require more than a year of work. In most cases, several programmers may work together as a team under a senior programmer's supervision. Applications programmers usually are oriented toward business, engineering, or science. They write software to handle specific jobs, such as a program used in an inventory control system or one to guide a missile after it has been fired.