Chapter 12: Regression Analysis
Complete Step-by-Step Solutions & Mathematical Reasoning
Check Your Progress 12.1
| Age (X) | 20 | 25 | 30 | 35 | 40 | 45 | 50 |
|---|---|---|---|---|---|---|---|
| Glucose Level (Y) | 60 | 72 | 75 | 80 | 85 | 90 | 93 |
Step 1: Calculate the sums of the variables.
Number of observations, .
Step 2: Calculate the regression coefficient Y on X ().
Step 3: Calculate the regression coefficient X on Y ().
| Mean | SD | |
|---|---|---|
| Capital employed (crores) | ||
| Profit earned (crores) |
(i) Find the two regression coefficients.
(ii) What is the average increase in profit for a unit increase in capital employed?
(iii) Which is more variable i.e., capital employed (X) or profit earned (Y)?
(i) Find the two regression coefficients.
Using the relation between correlation coefficient and standard deviations:
(ii) Average increase in profit:
The average change in profit (Y) for a unit increase in capital employed (X) is given by the regression coefficient of Y on X ().
Therefore, the average increase is 1.44 crores.
(iii) Which is more variable?
To compare variability, we calculate the Coefficient of Variation (CV) for both variables:
Since , Profit earned (Y) is substantially more variable.
, , , , ,
Step 1: Use the standard deviation method formula.
When deviations are given, the regression coefficients are calculated as:
Step 2: Substitute the provided values.
Since and , the formulas simplify directly to:
, , , , , .
It was later discovered that two pairs of observations (8, 12) and (6, 8) were wrongly copied as (6, 14) and (8, 6) respectively. Determine the correct value of regression coefficient .
Step 1: Identify the correct and incorrect pairs.
Correct pairs to ADD: (8, 12) and (6, 8)
Incorrect pairs to SUBTRACT: (6, 14) and (8, 6)
Step 2: Calculate the corrected statistical sums.
Step 3: Calculate the corrected regression coefficient .
, , ,
.
Which of the two variables i.e. X or Y changes more for unit change in the other?
Step 1: Simplify the data using substitutions.
Let and . The given data translates to:
, , , and .
Because and , we know that 60 and 79 are the actual arithmetic means and .
Step 2: Calculate the regression coefficients.
Using the direct formula for deviations from the mean:
Step 3: Compare to determine which variable changes more.
The coefficient indicates the change in Y for a unit change in X. The coefficient indicates the change in X for a unit change in Y.
Since (0.667 > 0.545), Y changes more for a unit change in X than X does for a unit change in Y.
, , , , , , .
Step 1: Find the number of observations (n).
We know that . Given and , we have:
Step 2: Calculate the regression coefficients using the raw score formula.
For Y on X:
For X on Y:
Practice Exercise (MCQs)
- 10
- 20
- 40
- 32
The regression line passes through the point . Substituting :
Price (p): 2, 4, 6, 8, 12
Quantity Supplied (x): 12, 15, 18, 21, 27
- –0.54
- 0
- 0.84
- 1
Observing the changes: universally across all data points. This denotes a perfect linear relationship () with a positive slope. Thus, the correlation coefficient is precisely 1.
- 4
- 7
- 12
- 20
Assume is on . Then ().
Assume is on . Then ().
Product . This is correct. Using on to predict at :
.
- 1.5
- 0.75
- –1.5
- –0.75
Regression coefficients scale proportionally: .
The scale of is 2, the scale of is -2. Given .
- 0.5
- 0.25
- –0.5
- –0.25
. The correlation coefficient shares the sign of the regression coefficients (negative).
- 1/5
- 1/4
- 1/2
- 4/5
From the equation , we have .
Using with :
- 3
- 4
- 6
- 12
Using the definition of the regression coefficient:
- 180
- 80
- 130
- 30
The intersection of the two regression lines gives .
From , we have .
Substitute into :
Now find : .
- intercept of the regression line.
- change in X for unit change in Y.
- coefficient of correlation.
- change in Y for unit change in X.
In a regression equation , the coefficient is the slope (). This explicitly represents the expected or average change in corresponding to a one-unit change in .
- 20
- 26
- 33.8
- 40
The regression line of on must pass through the point . Substitute to find :
- intercept of the regression line.
- change in X for unit change in Y.
- change in Y for unit change in X.
- coefficient of correlation.
The slope in the regression equation of Y on X (which is 0.65 here) mathematically describes the expected change in the dependent variable (Y) for every single unit increase in the independent variable (X).
Assertion-Reason Based Questions
Reason (R): The independent and dependent variables interchange in the two equations.
Step 1: Analyze Assertion (A).
The regression equation of X on Y is used to predict X given Y and yields the regression coefficient . The regression coefficient corresponds to the regression equation of Y on X. Therefore, you indeed cannot determine directly from the equation of X on Y alone. Assertion (A) is TRUE.
Step 2: Analyze Reason (R).
In the regression equation X on Y, Y acts as the independent variable and X as the dependent variable. In the equation Y on X, X is independent and Y is dependent. The roles interchange. Reason (R) is TRUE.
Step 3: Check Explanation.
Although both statements are true, the real reason we cannot determine from X on Y is that the two coefficients mathematically represent different slopes derived from minimizing different errors (horizontal vs vertical distances), not merely because the labels swap. Hence, (R) is true but not the direct, complete mathematical explanation for (A).
Reason (R): The angle between two regression lines is given by .
Step 1: Analyze Reason (R).
The mathematical formula for the angle between two lines of regression is indeed given by:
This statement is TRUE.
Step 2: Analyze Assertion (A).
If the lines are perpendicular (angle = ), then .
Looking at the formula in (R), the expression approaches infinity when the denominator is 0. That happens when , which means .
Thus, if the angle is , there is no correlation (). Assertion (A) is TRUE.
Reason (R): The coefficient of correlation is the geometric mean between the two regression coefficients.
Step 1: Analyze the Reason (R).
We know from the properties of regression analysis that the correlation coefficient () is exactly the geometric mean of the two regression coefficients ( and ).
This statement is factually TRUE.
Step 2: Analyze the Assertion (A).
Because the correlation coefficient always lies in the range , its square must be less than or equal to 1 ().
Therefore, the product of the regression coefficients must be:
If one regression coefficient (e.g., ) is strictly greater than 1, then for their product to remain less than or equal to 1, the other regression coefficient () must be strictly less than 1.
It is mathematically impossible for both regression coefficients to be greater than 1 simultaneously. Hence, the Assertion (A) is FALSE.
Reason (R): If one regression coefficient is less than one than the other regression coefficient is always greater than one.
Step 1: Analyze Assertion (A).
The signs of , , and are identically linked to the covariance between X and Y. Thus, they must all be positive or all be negative. Assertion (A) is TRUE.
Step 2: Analyze Reason (R).
We know that the product of the two regression coefficients is bounded by 1: .
If one is greater than 1, the other must be less than 1. However, if one is less than 1 (say 0.5), the other does NOT have to be greater than 1 (it could also be 0.5). Therefore, the statement "is always greater than one" makes Reason (R) FALSE.
Subjective Questions
| Marks in Statistics (X) | 25 | 35 | 32 | 31 | 36 | 34 | 42 | 45 |
|---|---|---|---|---|---|---|---|---|
| Marks in Mathematics (Y) | 43 | 44 | 49 | 41 | 35 | 30 | 32 | 46 |
(b) Find the most likely marks in statistics when the marks in mathematics is 48.
Let represent marks in Statistics and represent marks in Mathematics. Number of observations, .
Step 1: Calculate the arithmetic means.
Step 2: Construct the calculation table.
Let and .
| 25 | 43 | -10 | 3 | 100 | 9 | -30 |
| 35 | 44 | 0 | 4 | 0 | 16 | 0 |
| 32 | 49 | -3 | 9 | 9 | 81 | -27 |
| 31 | 41 | -4 | 1 | 16 | 1 | -4 |
| 36 | 35 | 1 | -5 | 1 | 25 | -5 |
| 34 | 30 | -1 | -10 | 1 | 100 | 10 |
| 42 | 32 | 7 | -8 | 49 | 64 | -56 |
| 45 | 46 | 10 | 6 | 100 | 36 | 60 |
| 280 | 320 | 0 | 0 | 276 | 332 | -52 |
(a) Calculating the coefficient of correlation ():
Using the formula for direct deviation method:
(b) Calculating the most likely marks in Statistics () when Mathematics () is 48:
We need to formulate the regression equation of on . First, find the regression coefficient :
The regression equation of on is given by:
Substitute the known values:
Now, to estimate when :
(b) Most likely marks in Statistics when Mathematics is 48 is . (Or if predicting Mathematics from Statistics).
| Height of mothers (X) | 148 | 149 | 150 | 152 | 153 | 155 | 157 | 160 |
|---|---|---|---|---|---|---|---|---|
| Height of daughters (Y) | 120 | 125 | 122 | 128 | 130 | 140 | 133 | 150 |
(b) Estimate the height of mother when the height of daughter is 145 cm.
Let represent height of mothers and represent height of daughters. Number of observations, .
Step 1: Calculate the arithmetic means.
Step 2: Construct the calculation table to find deviations.
Let and .
| 148 | 120 | -5 | -11 | 25 | 121 | 55 |
| 149 | 125 | -4 | -6 | 16 | 36 | 24 |
| 150 | 122 | -3 | -9 | 9 | 81 | 27 |
| 152 | 128 | -1 | -3 | 1 | 9 | 3 |
| 153 | 130 | 0 | -1 | 0 | 1 | 0 |
| 155 | 140 | 2 | 9 | 4 | 81 | 18 |
| 157 | 133 | 4 | 2 | 16 | 4 | 8 |
| 160 | 150 | 7 | 19 | 49 | 361 | 133 |
| 1224 | 1048 | 0 | 0 | 120 | 694 | 268 |
(a) Line of regression of mother () on daughter ():
First, find the regression coefficient :
The equation for the line of regression of on is:
(b) Estimating height of mother () when height of daughter () is 145 cm:
Substitute into the regression equation:
(b) Estimated height of mother is .
| X | 61 | 62 | 58 | 63 | 64 | 65 | 64 | 67 | 69 | 72 |
|---|---|---|---|---|---|---|---|---|---|---|
| Y | 112 | 115 | 108 | 118 | 120 | 125 | 119 | 125 | 130 | 140 |
Let represent height and represent weight. Number of observations, .
Step 1: Calculate the arithmetic means.
Step 2: Use Assumed Means to simplify calculations.
Let assumed mean for be and for be .
Define deviations: and .
| 61 | 112 | -3 | -8 | 9 | 64 | 24 |
| 62 | 115 | -2 | -5 | 4 | 25 | 10 |
| 58 | 108 | -6 | -12 | 36 | 144 | 72 |
| 63 | 118 | -1 | -2 | 1 | 4 | 2 |
| 64 | 120 | 0 | 0 | 0 | 0 | 0 |
| 65 | 125 | 1 | 5 | 1 | 25 | 5 |
| 64 | 119 | 0 | -1 | 0 | 1 | 0 |
| 67 | 125 | 3 | 5 | 9 | 25 | 15 |
| 69 | 130 | 5 | 10 | 25 | 100 | 50 |
| 72 | 140 | 8 | 20 | 64 | 400 | 160 |
| 645 | 1212 | 5 | 12 | 149 | 788 | 338 |
Step 3: Calculate the regression coefficient of Y on X ():
Since we need to predict weight () given height (), we compute using the shortcut formula:
Step 4: Formulate regression line Y on X and predict Y.
The equation is:
Substitute the requested height inches:
, , , , and
Step 1: Calculate the regression coefficients.
Using standard deviations, the regression coefficient of on () is:
The regression coefficient of on () is:
Step 2: Form the regression equation of Y on X.
Step 3: Form the regression equation of X on Y.
Line of X on Y:
| Rainfall (in inches) [X] | Production (per acre) [Y] | |
|---|---|---|
| Mean | 25 | 40 |
| Standard Deviation | 4 | 6 |
Step 1: Identify the variables and knowns.
Let be Rainfall and be Production Yield. We need to estimate given . We must find the regression equation of Y on X.
,
,
Step 2: Calculate .
Step 3: Construct the regression line and estimate . Substitute :
, , , , .
Find the equation of the line of regression and estimate the value of X when Y = 12.
Step 1: Understand the objective.
We are asked to estimate when is given. This requires the regression line of X on Y.
Step 2: Calculate the means.
Given .
Step 3: Calculate the regression coefficient .
Using raw data formula:
Step 4: Formulate the regression line and estimate X.
Equation of X on Y:
Substitute :
Step 1: Find the means by solving the equations simultaneously.
Since the regression lines intersect at , we solve:
(1)
(2)
Substitute (1) into (2):
Substitute into (1):
So, and .
Step 2: Identify the regression equations to find correlation ().
Assume is the line of Y on X. Rearranging for Y:
Thus, .
Assume is the line of X on Y. Rearranging for X:
Thus, .
Check validity: . Since , our assumption is correct.
Step 3: Calculate . Since both coefficients are positive, must be positive:
(i) Find the arithmetic means of x and y.
(ii) Identify the regression equation y on x.
(iii) Compute correlation coefficient between x and y.
(iv) Find the standard deviation of y, given the variance of x is 16.
(i) Arithmetic means:
Solve the system: (1) and (2) .
Substitute into (1):
So, . Therefore, , .
(ii) Identify regression equation of y on x:
Assume is y on x. Then , so .
Assume is x on y. Then , so .
. This is valid. The equation y on x is .
(iii) Compute correlation coefficient:
(Negative because coefficients are negative).
(iv) Find standard deviation of y:
Given .
We know .
(Alternatively, using : )
(ii) Equation y on x: .
(iii) Correlation coefficient .
(iv) Standard deviation of y is .
If the variance of is 25, find the standard deviation of .
Step 1: Identify the correct regression lines.
Assume is the line of Y on X. Rearranging gives . So .
Assume is the line of X on Y. Rearranging gives . So .
Product . The assumption is correct.
Step 2: Use the formula linking variances and regression coefficients.
We know:
Given :
Case Study Analysis
On the basis of the data provided in the original text (implied standard values corresponding to X_mean = 3.16, Y_mean = 4.28, and a regression line):
(i) Comment on the direction and degree of coefficient of correlation from the scatter diagram.
(ii) Find the regression equation of card charges (Y) on the travelled distance (X).
(iii) Predict the value of card charge (Y) if distance traveled is 6000 miles (X = 6).
(i) Direction and Degree:
If the scatter plot points are densely clustered along a straight line that slopes upwards from left to right, this indicates a strong, positive (or direct) correlation. As the distance travelled increases, the card charges also increase proportionally.
(ii) Regression Equation:
Using the provided standard answers from the text analysis, the regression equation of Y on X can be framed using the computed slope and means , :
(iii) Prediction:
Substitute (since X is measured in thousands of miles) into the equation:
Since Y is measured in thousands of dollars, the predicted charge is .
(ii) Equation: .
(iii) Predicted charge is $8060.
