Year 7 Science · Unit 2 · Lesson 11
Challenge Worksheet
Learning Goals
Compare reliability and validity
Complete both cells in each row.
| Feature | Reliability | Validity |
|---|---|---|
| What it means | ||
| How you improve it | ||
| The kind of error it deals with | ||
| Can you have it without the other? Give an example. |
Evaluate the investigation report
A student wrote this report
“I wanted to find out if the type of sugar affects how quickly it dissolves. I used icing sugar, raw sugar and white sugar. I added a spoonful of each to a cup of water and timed how long it took to dissolve. I used hot water for the icing sugar because it takes longer, and cold water for the other two. I only tested each sugar once because I ran out of time. I concluded that icing sugar dissolves fastest because its particles are more exposed. My experiment was a fair test because I used the same cup each time.”
1. Identify three flaws in the student’s method. For each flaw, say whether it reduces validity or reliability.
2. The student says: “My experiment was a fair test because I used the same cup each time.” What is wrong with this reasoning? What does a fair test actually need?
3. Icing sugar may well dissolve fastest. Is the student’s conclusion supported by this investigation? Explain using the ideas of validity and controlled variables.
Read the data: two groups, one test
Scenario: jelly cubes
Two groups did exactly the same test five times: one 10 g jelly cube in 200 mL of water at 60 °C, stirred at the same rate, timed until no jelly was left. Group B always finished stirring the cube in before starting the stopwatch, so they pressed start about 10 s late every time.
| Group | Trial 1 | Trial 2 | Trial 3 | Trial 4 | Trial 5 | Average |
|---|---|---|---|---|---|---|
| A | 118 | 131 | 109 | 125 | 122 | 121 |
| B | 110 | 111 | 109 | 110 | 110 | 110 |
Source: sample class results for this worksheet.
4. Which group’s results are more reliable? Use the spread of the trials in Table 1.
5. Which group’s average is likely to be closer to the true dissolving time? Explain, using the kinds of error in each group.
Plan an investigation: flat soda water
Question: Does the temperature of soda water affect how much mass it loses in the 10 minutes after the bottle is opened? The mass lost is mostly carbon dioxide escaping. You have six identical sealed bottles of soda water, an electronic balance, a stopwatch, a fridge (4 °C), the bench (20 °C) and a warm water bath (35 °C).
Hypothesis
Variables
Method
Results table: write the other column headings, with units
| Temperature (°C) | |||
|---|---|---|---|
| 4 | |||
| 20 | |||
| 35 |
A hidden problem: the bottles could lose mass in another way. Name it and say how to check for it.
6. A student says: “If I repeat my experiment ten times and get the same result every time, my result must be correct.” Evaluate this claim. Use Table 1 and one example of your own.
Compare reliability and validity
One mark per row, both cells right. Meaning: similar results when repeated / measuring what you intend, with only the IV changing (1). Improve: at least three trials and average / control every other variable and test the question directly (1). Error: random errors that scatter readings / errors that push every reading the same way, or uncontrolled variables, fixed in the method (1). Yes: scales always 10 g heavy are reliable, not valid; a well-controlled test with scattered times is valid, not reliable (1). [4 marks]
Evaluate the investigation report
1. Any three, 1 each with the correct label: hot water for one sugar only (validity); a “spoonful”, mass not controlled (validity); “a cup of water”, volume not measured (validity); each sugar tested once (reliability). [3 marks]
2. One control (the cup) is not enough; temperature, mass and volume also changed (1). A fair test changes only the IV and keeps every other variable the same (1). [2 marks]
3. No (1). Only the icing sugar was in hot water, so its speed could be caused by temperature, not sugar type (1). Mass and volume were not controlled either, so the test is not valid and cannot support the conclusion, even if it happens to be true (1). [3 marks]
Read the data: two groups, one test
4. Group B (1): 109 s to 111 s (2 s spread) vs Group A’s 109 s to 131 s (22 s) (1). [2 marks]
5. Group A’s (1). A’s errors are random, some high and some low, so averaging five trials cancels them (1). B’s late start makes every reading about 10 s short; averaging cannot remove that, so 110 s is about 10 s too low (1). [3 marks]
Plan an investigation: flat soda water
Hypothesis: if the soda water is warmer, it will lose more mass in 10 minutes, because gases are less soluble in warm water (1). IV: temperature (4, 20, 35 °C); DV: mass lost in 10 minutes (g); controlled, any three: brand and volume, bottle type, opened the same way, not shaken, same balance (1). Method: e.g. 1. Keep two bottles at each temperature until they reach it. 2. Open one, put it on the balance with its cap and record the mass. 3. Record the mass after 10 minutes and subtract. 4. Repeat with the second bottle and average (1 mass before and after, 1 repeats). Headings: Trial 1 mass lost (g); Trial 2 mass lost (g); Average (g) (1). Hidden problem: evaporating water also loses mass, faster when warm; check with opened bottles of plain water at each temperature (1). [6 marks]
Question 6
Marking guide: 1 clear position (wrong); 1 agreeing repeats show reliability; 1 averaging cancels random errors, not one that pushes every reading the same way; 1 Group B: five readings within 2 s, all about 10 s short; 1 a second example, e.g. a thermometer that always reads 3 °C high; 1 a correct result needs a valid method: other variables controlled, measurement made properly. [6 marks]
Model answer: The claim is wrong: ten matching results show reliability, not correctness. Averaging cancels random errors but not one that pushes every reading the same way. Group B agreed within 2 s, yet all were about 10 s short. A thermometer reading 3 °C high is wrong the same way every time. Only a valid method, measured properly with other variables controlled, gives a correct result.
Common error: treating close agreement as proof a result is correct.