The sample of telephone exchanges called was selected by a computer from a list of all city exchanges.

The sample of telephone exchanges called was selected by computer from a list of all exchanges in the United States.

The samples of telephone exchanges called were selected by a computer from complete lists of exchanges in Pennsylvania and the nation.

The sample of telephone exchanges called was randomly selected by a computer from a complete list of more than 42,000 residential exchanges across the country.

The telephone exchanges called were selected by a computer from a complete list of exchanges in the country.

The sample of telephone exchanges called was selected by a computer to assure that each region of the country was represented in proportion to its population.

The samples of telephone exchanges called were randomly selected by a computer from complete lists of active residential exchanges across the country and throughout New York City.

The telephone exchanges called were randomly selected by a computer from a complete list of more than 42,000 active residential exchanges in the United States.