Problem Solving .

Experimental Design

Reliability

An experiment is only reliable if it can be repeated with a similar result.

Repeating the experiment makes sure that the result was not an accident at the extreme end of the normal range.

Source: Office for National Statistics; General Register Office for Scotland; Northern Ireland Statistics and Research Agency

If you selected one person in the year 2000 who had died accidentally it is unlikely that you would have chosen a woman in the age group 0 to 15 but it could have happened. To be sure of the reliability of your data you must sample many people.

Validity

An experiment is only valid if all the variables apart from the one you are changing are kept the same.

In an experiment to measure the rate of digestion of starch by amylase at different temperatures you would need to make sure all the following variables were kept the same to make the procedure valid:

• pH
• concentration and volume of amylase solution
• concentration and volume of starch solution
• shape of container
• agitation
• time

Control Experiment

An experiment is only valid if it has a suitable control experiment:

• a control experiment is to make sure that the factor that you are investigating is the factor that is causing the effect.

In an experiment to measure the rate of digestion of starch by amylase at different temperatures you would need to repeat the experiment with boiled (destroyed, denatured) amylase.

• The variable you are changing is the temperature
• If you did not repeat with denatured amylase it could have been the temperature alone that was causing the effect.

Averages

You calculate an average to get a better idea of the true value of a variable:

• In biology there is always experimental error
  • There are small differences in the way the experiment is carried out
  • there are variations in the organisms you use
  • there are small variations in the way results are measured/collected
• The more values you collect and average the better the result is likely to be
• To calculate an average:
  • add up all the values
  • divide the total by the number of values that you summed

For example:

Total number of leaves = 30, there are 5 plants, 30 ÷ 5 = 6

Percentages

A percentage is a type of fraction.

The only difference between 3/4 (tree quarters) and 75% is that 75% is 75/100 (seventy five hundredths).

So 3 out of 4 is the same as 75 out of 100