1. Evaluative practice What is Evaluation? The consideration of:
The way the experiment was carried out
The quality of the data
and also: the processing of that data

2. The Experiment – Validity (or did it measure what you wanted it to?)
Limitations
Uncontrolled variables
Results
Effects of errors (systematic and random)
Reliability
Repeatability/reproducibility
Accuracy
Precision

3. Definitions 1. Accuracy 2. Precision 3. Reliability 4. Validity
How close our measurements are to the true value
Repeated values show little variation
If the test is repeated and the same results are obtained
Does the experiment measure the correct variable
The variable that is measured to see the response to the change in the independent variable.
The variable that we change or manipulate
1. Accuracy
2. Precision
3. Reliability
4. Validity
5. Independent variable
6. Dependent variable
1E, 2D, 3A, 4B, 5F, 6C

4. Limitations Limitation How limitation affects method or results
Example of how to overcome limitation
Insufficient intermediate readings taken e.g. every 60 sec.
Could miss changes occurring between intervals measured.
Take readings at smaller intervals eg 30 sec instead of 60 sec.
Insufficient range of independent variable; e.g OC.
Lack of results beyond range – makes determining trend difficult.
Extend the range, eg 10-80OC.
Insufficient number of values for independent variable.
Makes determining trend difficult.
Include more intermediate values.
One measurement or only two replicates.
Unable to detect anomalous results, decreases reliability.
Perform at least three replicates.
Difficulty in judging colours or changes when using ‘by eye’ methods.
Lack of consistency in judgement causing less reliable, precise and accurate results.
Use colour standards, colorimeter, time movement of ball bearing for coagulation or any standard method for comparing change.
Inconsistent stirring of solutions.
Inconsistent rate of product formation or separation /non-mixing of components of solution.
Standardise method and time of stirring.
Samples from different sources.
Different age/state may introduce random errors
Use same source for samples
Method of timing.
Used to a greater/lesser level of precision than sensible (e.g 0.01s)
Record times to nearest second, 10 seconds or whatever is appropriate.
Impossible to start all reactions at same time.
Some reactions occurred for longer or shorter times.
Use a staggered start, e.g.at 1 minute intervals
pH not controlled.
pH affects enzyme activity.
Use a buffer solution to control pH.

5. Systematic Errors
Errors that are the same throughout the investigation.
e.g. A measuring device that is ‘out’ by a certain value, one of the controlled variables is always incorrect by the same quantity.
Results may still be precise, but not accurate.

6. Random Errors
Occur when procedure not carried out in the same way each time it is performed.
e.g. Reading the apparatus differently every time, using any of the equipment a different way each time or due to variations in biological material used in experiment.
Some results will be affected, but not all. May cause one or more anomalous results.

7. Processing results Calculating mean, median, mode
Mean – average of all data collected. (add all values up and divide by number of results)
Median – middle value when all collected data listed in order of size. For even number of results, find the mid point of the two middle values.
Mode – The most common data value. (often tally or bar chart helps to identify)
Range – Difference between the smallest and largest values, or between the replicates of an experiment.
Calculating reaction rate (1/t, or depending on size of number, 100/t or 1000/t; t=time taken for reaction)
Calculating relative reaction rate (Choosing one reaction (reference) and comparing the other reactions to it – time taken for reference reaction to occur/ time taken for other reaction).
Graphs:
Use range bars to show range of results around the mean
Calculate standard deviation (not till A2) but can use provided SD values
for error bars on graph, the SD is plotted above and below the point).

8. Rate Calculations
Relative Rate (different to rate of reaction, see previous slide)
If you compare your reaction rate to one of the other reactions.... Say 10mmol is the maximum (reference) in this experiment. You wish to calculate the relative speed of the other reactions to this one.
So: 10 (sec) / 20 sec (for 8mmol) = 0.5 relative rate
OR, percentage relative activity of enzyme:
Enzyme activity is at 100% at 4mmol
So for 8mmol percentage relative activity is
40 (sec for 4mmol)/20 (sec for 8mmol) *100
= 200%
Enzyme concentration (mmol)
Time taken (s)
Relative rate 2 60
0.16 4 40
0.25 6 30
0.33 8 20
0.50 10
1.00

9. Evaluate your experiment (do this sheet at home after your quantitative!)
Brief background to experiment
What did the experiment intend to show?
Could you formulate a hypothesis?
What limitations were there?
What effect on the results?
How could the method be changed to overcome the limitation?
Is there some way to process your data to show
how significant/reliable it is?
What was the data like? Were there any anomalies? Why?
What conclusion could you draw from your results?
What evidence do you have to support this conclusion?