Foundation November 2020 Paper 2 Q22
22 A weather forecast says
- the probability that it will rain on Monday is 0.55
and
- the probability that it will rain on Tuesday is 0.25.
Ella draws a tree diagram to show this information.
Write down three errors that Ella has made with her tree diagram. [3]
| Answer | Marks | Part marks and guidance | |
|---|---|---|---|
| For Monday, does not rain should be 1 – 0.55 oe For Tuesday, 0.25 is incorrectly placed on the does not rain branch oe A pair of branches is missing for Tuesday after does not rain on Monday oe | 3 | B1 for each | After each correct statement isw eg 0.55 + 0.35 does not equal 1 Monday not rain should be 0.45 eg For Tuesday the probabilities are placed the wrong way around 0.25 should be on the rain branch eg There should be two more branches for Tuesday See AG |
Exemplar responses
Mark clear intention and condone slips in language provided intention is clear. e.g. accept tree for branches
Exemplar responses for Q22
| Response | Mark |
|---|---|
| 0.35 needs to be 0.45 | 1 |
| She didn’t subtract the 0.55 probability that it rain from 1 to get the probability that it doesn’t rain (0.45 seen on the diagram and 0.35 crossed out) | 1 |
| The (0.35) they did not show it, how did they get that number, because it’s wrong (error identified) | 1 |
| Don’t add to 100 on Monday (condone lack of % sign) | 1 BOD |
| They do not add up to 1 (but if 0.55 and 0.35 shown then this would score) | 0 |
| A probability tree always adds up to 1 (but if ref to 0.55 and 0.35 then this would score) | 0 |
| 0.55 + 0.35 = 90, meaning she’s not taking it from 1 (statement incorrect) | 0 |
| For Tuesday it should be 0.25 for rain and 0.75 for not rain (if stated as separate reasons this is just 1 mark) | 1 |
| She says probability that it rains on Tuesday is 0.75 but it is 0.25 | 1 |
| She has said that the probability it doesn’t rain on Tuesday is 0.25. (points out the error) | 1 |
| On Tuesday the chance of rain is 0.25 (just restating the stem needs further explanation) | 0 |
| For Tuesday the probability it will rain is plotted wrong (not specific enough) | 0 |
| In the second tree diagram, she has the wrong number for rain (not specific enough) | 0 |
| She wrote the probability “0.25” that it rains on Tuesday in the wrong section on the diagram (not specific enough) | 0 |
| There would be another tree diagram ( with two more branches correctly drawn on the diagram ) | 1 |
| She only drew 2 trees, she should have shown the probability of it raining and not raining on each end of the tree | 1 |
| She has put Tuesday branch following on from Monday it rains and has not done the Monday it does not rain tree | 1 |
| There is no tree for Tuesday it does not rain (BOD with position indicated) | 1 |
| She needs a second tree diagram for it does not rain (or for Tuesday) | 1 |
| There is no second branch for Tuesday, (or there is not a Tuesday for it does not rain) | 1 |
| She’s not continued the does not rain section (BOD gives some indication of position) | 1 |
| There should be another branch (Needs to indicate where) | 0 |
| She doesn’t have all the branches that are needed | 0 |
| She hasn’t completed the whole tree diagram with all the outcomes | 0 |