Look at the formula in the Example tab for Number of defective CFLs:

=COUNTIF(B4:B153,0). This formula scans the values in cells B4 through B153 (life spans) to check whether the value is 0. “IF” so, it “COUNTS” it (adds 1 to a total).

Return to the Test Sample tab, and enter a formula like this into cell E5. How does the resulting value compare with your hand count of defective bulbs?

Again on the Test Sample tab, enter a spreadsheet formula to calculate the proportion of defective CFLs in the test sample: the ratio of defective bulbs as a fraction of the sample. Write down the formula you used and the result you got.