40. Hexavalent chromium has been identi ed as an inhalation carcinogen and an air toxin of concern in a number of different locales. The article Airborne Hexavalent Chromium in Southwestern Ontario (J.

Air Waste Manag. Assoc., 1997: 905—910) gave the accompanying data on both indoor and outdoor concentration

(nanograms/m3) for a sample of houses selected from a certain region.

House 1 2 3 4 5 6 7 8 9 Indoor .07 .08 .09 .12 .12 .12 .13 .14 .15 Outdoor .29 .68 .47 .54 .97 .35 .49 .84 .86 House 10 11 12 13 14 15 16 17 Indoor .15 .17 .17 .18 .18 .18 .18 .19 Outdoor .28 .32 .32 1.55 .66 .29 .21 1.02 House 18 19 20 21 22 23 24 25 Indoor .20 .22 .22 .23 .23 .25 .26 .28 Outdoor 1.59 .90 .52 .12 .54 .88 .49 1.24 House 26 27 28 29 30 31 32 33 Indoor .28 .29 .34 .39 .40 .45 .54 .62 Outdoor .48 .27 .37 1.26 .70 .76 .99 .36

a. Calculate a con dence interval for the population mean difference between indoor and outdoor concentrations using a con dence level of 95%, and interpret the resulting interval.

b. If a 34th house were to be randomly selected from the population, between what values would you predict the difference in concentrations to lie?