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Evaluation skills help sheet by zU8n82jP


									                                                  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 =

                          Record your answer to 1 decimal place.

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
                                                                                   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

                          conclusions about the data.

                             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

                                         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
      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
         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

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

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