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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 = 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 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 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 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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