A large department store wishes to carry out a survey by issuing a questionnaire to a sample of 100 of its 2,000 account holders. The sample is to be selected as follows: Each customer will be allocated a number from 1 to 2000. A table of random numbers from 1 to 20 will then be used to select the first member of the sample. Every succeeding 20th member of the population of account holders will then be selected. This form of sampling is called _____________________.