We have a job with N tandem tasks each of which is executed successively until it is completed. A double modular system of error detection for the processing of each task is adopted. Either type of checkpoints such as compare-checkpoint or compare-and-store-checkpoint can be placed at the end of tasks. Three schemes for the above process of a job are considered and the mean execution time of each scheme is obtained. Three schemes are compared and the best scheme is determined numerically. As an example, a job with 4 tasks is given and 6 types of schemes are compared numerically. Finally, we consider a majority decision system as an error masking system and compute the mean execution time for three schemes.