S3 June 2012 Q2
2. A lake contains 3 species of fish. There are estimated to be 1400 trout, 600 bass and 450 pike in the lake. A survey of the health of the fish in the lake is carried out and a sample of 30 fish is chosen.
(a) Give a reason why stratified random sampling cannot be used. (1)
(b) State an appropriate sampling method for the survey. (1)
(c) Give one advantage and one disadvantage of this sampling method. (2)
(d) Explain how this sampling method could be used to select the sample of 30 fish. You must show your working. (4)
| Scheme | Marks |
|---|---|
| Sampling frame within each species of fish in the lake impossible to obtain. | B1 |
| (1) |
Notes
‘You can’t / it’s very difficult to number all the fish’ or equivalent
| Scheme | Marks |
|---|---|
| Quota sampling | B1 |
| (1) |
| Scheme | Marks |
|---|---|
| Advantages: Sample can be obtained quickly Costs are kept to a minimum Administration of survey is easy | B1 |
| Disadvantages: Not possible to estimate sampling errors Process not random Surveyor may not be able to identify species of fish easily | B1 |
| (2) |
Notes
Correct answer to (b) required. Some detail required.
| Scheme | Marks | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| B1B1B1 | ||||||||
| If a fish is caught and the species quota is full, then this is ignored. | B1 | ||||||||
| (4) | |||||||||
| (8 marks) |
Notes
1st B1 any one correct calculation seen or implied
2nd B1 all correct to at least 1 dp
3rd B1 for 17,7,6
4th B1 accept equivalent statement. Require comment on what to do with ‘extra fish’.