A very well-known consumer publication has recently gathered some data concerning the life span of standard-size refrigerators. The publication reported the life span of 40 refrigerators chosen at random from the records of various manufacturers. The life spans (rounded to the nearest year) are listed below.

11	18	18	13	18
21	20	17	16	19
9	10	19	19	20
23	19	12	14	17
19	19	24	18	20
15	17	18	21	21
20	21	8	16	19
21	19	19	23	18

Construct a 95% confidence interval for the population average life span of standard-size refrigerators. Interpret your interval.