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# Evaluation skills help sheet by zU8n82jP

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```									                                                  Evaluation skills help sheet

1. Calculate the mean of the results 5, 8, 7, 4, 6

2. Use the formula to calculate the standard deviation of 5, 8, 7, 4, 6 Hint: set up

a table to help with each part of the calculation

Standard deviation =

3. This graph is based on 3 milk samples containing different levels of protein that

is being digested. Plot error bars on the graph using the standard deviation

shown in the table:

Milk      Standard
8                                                              sample     deviation
7                                                                A           0.8
time to go clear (sec)

6                                                                B           1.2
5                                                                C           0.4
4
3
2
1
0
Fig 1: Graph to show the
A                  B                C
milk sample                       time taken for protease to
digest milk

4. Use your graph, including the information from the error bars to draw

   Which sample is digested fastest and which is slowest?

   What does this suggest about the protein content of each milk sample?

   Can you be confident about the reliability of the data? Link to size of error

bars, do the error bars overlap?
5. Look at the results for sample A. How do you know there are no anomalous

results?
A       5.6        6.0    6.5

6. Why is the protein in caws milk likely to be different from goat’s milk?

7. What is UHT milk? Why is it important not to use UHT milk in this
experiment?
8. What is a control variable? Name 2 variables in this investigation that should be
controlled? Why should they be controlled? How could you control them?

9. An error is not a design fault but a single or ‘one off’ event that makes the data
unreliable.
Link the source of error to the correct explanation:

Source of error                       explanation
Contaminating syringe by              Time taken altered as
not washing out between               different volume of milk
milk samples                          present
Volume of milk not                    Alters surface area to
measured carefully                    volume ratio
Different size boiling tube           Protein concentration of
used for 1 test                       milk sample may be altered

10 A limitation is a design fault where some factors have not been controlled or taken
into account. Decide which if the following is an error and which is a limitation:

Error or            Effect on data
limitation?         collected
Hard to judge end point
Droplet size may vary
Temperature of room not constant
Contamination of solutions used
pH of sample changes during
experiment

11.      What is meant by density? What factors affect density?

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