Worked solution
Choose the regression line before substituting
Eight observed pairs are:
| 3 | 7 |
| 4 | 8 |
| 6 | 9 |
| 10 | 11 |
| 11 | 20 |
| 12 | 19 |
| 15 | 21 |
| 17 | 22 |
It is given that , the regression of on is , and the regression of on is .
(a) Taking as the independent variable, estimate (i) when and (ii) when .
(b) Now taking as the independent variable, estimate (i) when and (ii) when .
(c) With neither variable identified as independent, estimate (i) when and (ii) when .
(d) Use an appropriate regression line to estimate if the independent variable is correctly determined as 30. Explain why this is not a good estimate.
CJC · Lecture · Example 7
(a)(i) Keep x as the explanatory variable
Answer
.
The source explicitly selects on for case (a). Substitute into the given model: . Reported here to 3 s.f.; the visible numerical working is blank.
(a)(ii) Solve the same fixed model backwards
Answer
.
Because is still prescribed explanatory, use the same and solve . Thus . This authored blank completion is not the distinct -on- fitted estimate.
(b)(i) Keep y as explanatory
Answer
.
The applicable fit is now on . The source prints ; completing its blank calculation gives .
(b)(ii) Substitute into the reverse fit
Answer
.
The printed calculation is to 3 s.f. Before rounding it is . Here is the prescribed explanatory input.
(c)(i) Predict the requested response when roles are not fixed
Answer
Use on : .
Neither explanatory role is prescribed, so predicting from a supplied calls for the -on- fit. Substitute to get . This completes the blank source choice and calculation.
(c)(ii) Fit the other response for the other task
Answer
Use on : .
For a supplied and requested , use the -on- fit: . This completes the blank source choice and calculation; it differs from case (a)(ii) because the modelling roles differ.
(d) Identify the out-of-range explanatory value
Answer
, but this extrapolation is unreliable.
With explanatory use , giving . The numerical completion is authored. The source explicitly notes that lies outside the observed , so this is extrapolation. Check the range of the explanatory input , not only the range of the resulting . A high sample correlation does not establish the relationship outside that input range.
Solution notes
All seven asks, eight pairs, both supplied rounded fitted lines and r retained. Source provides selection reasoning, (b)(ii) and the extrapolation reason; blank numerical/choice completions authored. No printed marks; use the equations given rather than silently replacing them with refitted coefficients.