EVALUATIONS

Why have you performed a step in the procedure? What is the purpose of the step? Is this step required because of a previous stage or procedure? What would happen if you didn’t do it? How might the results be affected?

ACCURACY Accuracy relates to how close your observed or measured value is to the correct/real value. Accuracy can be improved by changing the experimental technique e.g. in a photosynthesis investigation by collecting oxygen over a longer time period and using a water bath to control temperature.   Measuring pulse rate in beats per minute Method 1: Count your pulse for 6 seconds and multiply by 10 Method 2: Count the number of beats in a minute Which method is likely to provide the most accurate data?

VALIDITY Validity is whether a test actually measures what it sets out to measure. If you are asked to comment on the validity of an investigation or to suggest ways in which the validity could be improved you should try to identify factors that are not or cannot be controlled and which may therefore be limiting the validity of the investigation.   E.g. Testing the effect of exercise on heart rate One student measure heart rates of 100 students from year 7 boys on a treadmill The other student measure the heart rate of 10 friends and family ranging from 5 to 90 years old Which data will be have the most validity in answering this question? Explain why

RELIABILITY Reliability relates to the consistency in (or your confidence with) a set of measurements. Reliability is improved by repetition - if repeated results lie close together with a small range then the results have high reliability. Taking repeat measurement also helps you identify and omit anomalous results - those that are clearly out of line with the other data. You would need at least 3 repeats to spot an anomalous result. More repeats give the option of calculating a standard deviation from the mean. This tells you how closely the data is clustered around the mean

RELIABILITY (cont) Large standard deviations mean that there is a lot of variation within the repeats This may mean that your method has design flaws or a number of variables that have not or may not be controlled Range bars can identify the smallest and largest values from the mean. The more overlap there is in the range bars the less difference there is between sets of data

Limitation Any factor that has not been controlled or taken into account in the experimental design. It may be a design fault What limitations could there be in the following: Testing the effect of pH on catalase found in potato disks Measuring the effect of caffeine on heart rate in the general population The effect of temperature on transpiration rate (using a hairdryer on a plant) Measuring the abundance of limpets on the rocky shore to the distance from low tide line The change in mass of potato sticks in 3 different salt concentrations

Limitations continued If asked to suggest some then try to think of ones that you can explain, or improve upon. Some stages in a practical are not limitations if every test is carried out in the same way as the results have had the same treatment and are comparable.

General Limitations Living/plant tissues are naturally variable so consider this one. Repeats Intermediate values (not wider range) Methods of measuring the dependent variable.. are they subjective? (affected by interpretation such as a colour change) Control of other variables such as temperature, pH, size and shape of samples, stirring of solutions,

Errors These are mistakes May be things like plotting points on a graph incorrectly, calculation error, axis scaled incorrectly