The Results
The volunteers were assigned identical rooms, fed identical food, and came into hand contact with identical items. By controlling all the variables that might influence hand bacterial growth, the student felt sure that the only difference in the bacterial count after hand washing would be the type of soap used.
He measured the bacterial count before hand washing (A) and after hand washing (B) daily for 7 days and took an average count each day.
|
Average Number of Bacteria per Hand (x 1,000)
|
| Type of Soap |
Day 1
(2nd day)
|
Day 3
|
Day 4
|
Day 5
|
|
A
|
B
|
A
|
B
|
A
|
B
|
A
|
B
|
| Regular Soap |
1000
|
100
|
1000
|
100
|
1000
|
100
|
1000
|
100
|
| Antibacterial Soap |
1000
|
1
|
1000
|
10
|
1000
|
30
|
1000
|
200
|
| Control |
1000
|
500
|
1000
|
500
|
1000
|
500
|
1000
|
500
|
The numbers indicate the average bacterial counts between the regular soap and antibacterial soap differed. The antibacterial soap did kill 99.9% of the hand bacteria on Day 1. However, something unexpected happened. By Day 7, the bacterial count for the antibacterial soap group was higher than the regular soap group! Why?
From this data, the student made the following conclusions: “My hypothesis that antibacterial soap is no more effective at killing bacteria than regular soap is not supported.”
Other conclusions the student made were: “Although the antibacterial soap killed more bacteria than the regular soap during the first 5 days, by Day 7, this was no longer the case."
