A student wanted to investigate the transpiration rate of a plant species at varying levels of humidity. The results are recorded in the table shown. Each plant group was given
the same amount of water and light throughout the investigation which procedures should have been performed to prevent error in this investigation? (N 11)
Comparison of Transpiration Rates Over a 60 Hour Period
Plant Species
Average
Group
Humidity (96)
Type of Soil Transpiration
(10 Plants (200 g)
Rate
Each)
(g/dm/hr)
35
Group 1
Sand
23.5
46
Group 2
Sand
20.4
58
Group 3
Clay
18.8
20
Group
Clay
14.4
Both groups should have been tested in the same soil type at each humidity level.
The percentage of humidity should have been kept constant
Fewer plants should have been included in each group
Another plant species should have been tested

From the experimental process described above, the student failed to control for bias which could creep in by not controlling for the soil type used, controlling for possible error as a result of the soil type requires that each group is tested using the same soil type, this way, the student can be confident that variation in the result is not due to the non-uniformity in soil type the plants were subjected to. At each humidity level, the groups should be tested using the same soil type. This way, observed result would have been largely due to different humidity levels.