Class 11- Applied Mathematics-NCERT Handbook Solutions-Chapter-12 (Regression Analysis)

Class 11 Applied Mathematics - Chapter 12 Solutions

Chapter 12: Regression Analysis

Complete Step-by-Step Solutions & Mathematical Reasoning

Check Your Progress 12.1

Q1
Find the two regression coefficients byx and bxy for the following data:
Age (X)20253035404550
Glucose Level (Y)60727580859093
Solution

Step 1: Calculate the sums of the variables.
Number of observations, n=7.
X=20+25+30+35+40+45+50=245
Y=60+72+75+80+85+90+93=555
X2=202+252+302+352+402+452+502=9275
Y2=602+722+752+802+852+902+932=44783
XY=(20×60)+(25×72)+(30×75)+(35×80)+(40×85)+(45×90)+(50×93)=20150

Step 2: Calculate the regression coefficient Y on X (byx).
byx=nXY-(X)(Y)nX2-(X)2 byx=7(20150)-(245)(555)7(9275)-(245)2 byx=141050-13597564925-60025=507549001.036

Step 3: Calculate the regression coefficient X on Y (bxy).
bxy=nXY-(X)(Y)nY2-(Y)2 bxy=7(20150)-(245)(555)7(44783)-(555)2 bxy=< 141050\ -\ 135975\ }{\ 313481\ -\ 308025\ }=\frac{5075}{5456}\approx 0.930

The two regression coefficients are byx1.036 and bxy0.93.
Q2
Given below is the data regarding capital employed (X) and profit earned (Y) by an automobile parts company over the last two decades.
MeanSD
Capital employed (crores)X¯=72σx=5
Profit earned (crores)Y¯=3σy=8
Coefficient of correlation r=0.9.
(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)?
Solution

(i) Find the two regression coefficients.
Using the relation between correlation coefficient and standard deviations: byx=r×σyσx=0.9×85=0.9×1.6=1.44 bxy=r×σxσy=0.9×58=0.9×0.625=0.5625

(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 (byx).
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: CV=SDMean×100 CVx=572×1006.94% CVy=83×100266.67% Since CVy>CVx, Profit earned (Y) is substantially more variable.

Q3
Calculate the regression coefficients for the following data:
n=10, dx=0, dy=0, dx2=112, dy2=97, dxdy=82
Solution

Step 1: Use the standard deviation method formula.
When deviations are given, the regression coefficients are calculated as: byx=ndxdy-(dx)(dy)ndx2-(dx)2 bxy=ndxdy-(dx)(dy)ndy2-(dy)2

Step 2: Substitute the provided values.
Since dx=0 and dy=0, the formulas simplify directly to: byx=10(82)-010(112)-0=8201120=821120.732 bxy=10(82)-010(97)-0=820970=82970.845

The regression coefficients are byx0.732 and bxy0.845.
Q4
The regression coefficient byx is calculated on the basis of following data:
n=25, X=125, Y=100, X2=650, Y2=460, XY=508.
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 byx.
Solution

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.
Xnew=125-6-8+8+6=125
Ynew=100-14-6+12+8=100
Xnew2=650-62-82+82+62=650
XYnew=508-(6×14)-(8×6)+(8×12)+(6×8)
XYnew=508-84-48+96+48=508-132+144=520

Step 3: Calculate the corrected regression coefficient byx.
byx=nXYnew-(Xnew)(Ynew)nXnew2-(Xnew)2 byx=25(520)-(125)(100)25(650)-(125)2 byx=13000-1250016250-15625=500625=0.8

Note: While the textbook key lists 0.63, the correct mathematical derivation based on the provided data and stated errors incontrovertibly yields 0.8.
The correct value of the regression coefficient byx is 0.8.
Q5
Find the two regression coefficients from the following data:
(X-60)2=36, (Y-79)2=44, (X-60)(Y-79)=24,
(X-60)=0=(Y-79).
Which of the two variables i.e. X or Y changes more for unit change in the other?
Solution

Step 1: Simplify the data using substitutions.
Let x=X-60 and y=Y-79. The given data translates to:
x2=36, y2=44, xy=24, and x=y=0.

Because x=0 and y=0, we know that 60 and 79 are the actual arithmetic means X¯ and Y¯.

Step 2: Calculate the regression coefficients.
Using the direct formula for deviations from the mean: byx=xyx2=2436=230.667 bxy=xyy2=2444=6110.545

Step 3: Compare to determine which variable changes more.
The coefficient byx indicates the change in Y for a unit change in X. The coefficient bxy indicates the change in X for a unit change in Y.
Since |byx|>|bxy| (0.667 > 0.545), Y changes more for a unit change in X than X does for a unit change in Y.

The regression coefficients are byx0.667 and bxy0.545. The variable Y changes more.
Q6
Find the regression coefficient Y on X and X on Y on the basis of the following data:
X=50, X¯=5, Y=60, Y¯=6, XY=350, X2=300, Y2=420.
Solution

Step 1: Find the number of observations (n).
We know that X¯=Xn. Given X=50 and X¯=5, we have: 5=50nn=10

Step 2: Calculate the regression coefficients using the raw score formula.
For Y on X: byx=nXY-(X)(Y)nX2-(X)2 byx=10(350)-(50)(60)10(300)-(50)2=3500-30003000-2500=500500=1
For X on Y: bxy=nXY-(X)(Y)nY2-(Y)2 bxy=50010(420)-(60)2=5004200-3600=500600=560.833

The regression coefficient of Y on X is 1, and X on Y is 560.833.

Practice Exercise (MCQs)

Q1
Given, 8x+5y=240 is the regression line of x on y and the arithmetic mean of y is 32. What is the mean of x?
  • 10
  • 20
  • 40
  • 32
Solution

The regression line passes through the point (x¯,y¯). Substituting y¯=32: 8x¯+5(32)=2408x¯+160=2408x¯=80x¯=10

Correct Option: (a) 10
Q2
The coefficient of correlation between the given data:
Price (p): 2, 4, 6, 8, 12
Quantity Supplied (x): 12, 15, 18, 21, 27
  • –0.54
  • 0
  • 0.84
  • 1
Solution

Observing the changes: ΔxΔp=32=1.5 universally across all data points. This denotes a perfect linear relationship (x=1.5p+9) with a positive slope. Thus, the correlation coefficient r is precisely 1.

Correct Option: (d) 1
Q3
The two lines of regression are given by 9x+5y=74 and x+4y=34. What is the most likely value of y when x is 6?
  • 4
  • 7
  • 12
  • 20
Solution

Assume x+4y=34 is y on x. Then y=-0.25x+8.5 (byx=-0.25).
Assume 9x+5y=74 is x on y. Then x=-59y+749 (bxy=-59).
Product r2=(-0.25)×(-59)=5361. This is correct. Using y on x to predict y at x=6:
6+4y=344y=28y=7.

Correct Option: (b) 7
Q4
If u=2x+5 and v=-2y-3 and the regression coefficient of y on x is 1.5, then what is the regression coefficient of v on u?
  • 1.5
  • 0.75
  • –1.5
  • –0.75
Solution

Regression coefficients scale proportionally: bvu=scale of vscale of u×byx.
The scale of u is 2, the scale of v is -2. Given byx=1.5.
bvu=-22×1.5=-1×1.5=-1.5

Correct Option: (c) –1.5
Q5
The two regression coefficients i.e. bxy and byx are –2 and –1/8 respectively. The coefficient of correlation is:
  • 0.5
  • 0.25
  • –0.5
  • –0.25
Solution

r=±byx×bxy. The correlation coefficient shares the sign of the regression coefficients (negative).
r=-(-2)×(-18)=-28=-0.25=-0.5

Correct Option: (c) –0.5
Q6
If 4y-5x=15 is the regression line y on x and the coefficient of correlation is 0.5, what is the value of the regression coefficient x on y?
  • 1/5
  • 1/4
  • 1/2
  • 4/5
Solution

From the equation 4y=5x+15, we have byx=54.
Using r2=byx×bxy with r=0.5:
(0.5)2=14=54×bxybxy=14×45=15

Correct Option: (a) 1/5
Q7
The value of standard deviation of y given r=0.8, standard deviation of x=5 and byx=0.64 is:
  • 3
  • 4
  • 6
  • 12
Solution

Using the definition of the regression coefficient: byx=rσyσx
0.64=0.8×σy5 σy=0.64×50.8=3.20.8=4

Correct Option: (b) 4
Q8
The mean value of X given the two regression equations are 2Y-X=50 and 3Y-2X=10 is:
  • 180
  • 80
  • 130
  • 30
Solution

The intersection of the two regression lines gives (X¯,Y¯).
From 2Y-X=50, we have X=2Y-50.
Substitute into 3Y-2X=10: 3Y-2(2Y-50)=103Y-4Y+100=10Y=90 Now find X: X=2(90)-50=180-50=130.

Correct Option: (c) 130
Q9
In the regression equation Y on X i.e., Y=32+1.8X, where Y is in Fahrenheit and X is in Celsius, the coefficient of X i.e. 1.8 represents:
  • intercept of the regression line.
  • change in X for unit change in Y.
  • coefficient of correlation.
  • change in Y for unit change in X.
Solution

In a regression equation Y=a+bX, the coefficient b is the slope (byx). This explicitly represents the expected or average change in Y corresponding to a one-unit change in X.

Correct Option: (d) change in Y for unit change in X
Q10
Given the regression line X on Y is X=0.5Y+10, and the mean of Y is 20, what is the mean of X?
  • 20
  • 26
  • 33.8
  • 40
Solution

The regression line of X on Y must pass through the point (X¯,Y¯). Substitute Y¯=20 to find X¯:
X¯=0.5(20)+10=10+10=20

Correct Option: (a) 20
Q11
The slope of the regression equation Y on X i.e., Y=11.9+0.65X can be interpreted as:
  • intercept of the regression line.
  • change in X for unit change in Y.
  • change in Y for unit change in X.
  • coefficient of correlation.
Solution

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).

Correct Option: (c) change in Y for unit change in X

Assertion-Reason Based Questions

Q12
Assertion (A): Regression coefficient byx can not be determined from regression equation X on Y.

Reason (R): The independent and dependent variables interchange in the two equations.
Solution

Step 1: Analyze Assertion (A).
The regression equation of X on Y is used to predict X given Y and yields the regression coefficient bxy. The regression coefficient byx corresponds to the regression equation of Y on X. Therefore, you indeed cannot determine byx 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 byx 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).

Both (A) and (R) are true but (R) is not the correct explanation of (A). The correct option is (b).
Q13
Assertion (A): If the angle between the two regression lines is 90 then r=0.

Reason (R): The angle between two regression lines is given by tanθ=1-r2|r|(σxσyσx2+σy2).
Solution

Step 1: Analyze Reason (R).
The mathematical formula for the angle θ between two lines of regression is indeed given by: tanθ=1-r2|r|(σxσyσx2+σy2) This statement is TRUE.

Step 2: Analyze Assertion (A).
If the lines are perpendicular (angle = 90), then tanθ=tan90=.
Looking at the formula in (R), the expression approaches infinity when the denominator is 0. That happens when |r|=0, which means r=0.
Thus, if the angle is 90, there is no correlation (r=0). Assertion (A) is TRUE.

Since both are true and R is the mathematical derivation for A, Both (A) and (R) are true and (R) is the correct explanation of (A). The correct option is (a).
Q14
Assertion (A): If regression coefficient Y on X is greater than 1, then regression coefficient of X on Y must also be greater than 1.

Reason (R): The coefficient of correlation is the geometric mean between the two regression coefficients.
Solution

Step 1: Analyze the Reason (R).
We know from the properties of regression analysis that the correlation coefficient (r) is exactly the geometric mean of the two regression coefficients (byx and bxy). r=±byx×bxy This statement is factually TRUE.

Step 2: Analyze the Assertion (A).
Because the correlation coefficient r always lies in the range [-1,1], its square r2 must be less than or equal to 1 (r21).
Therefore, the product of the regression coefficients must be: byx×bxy1 If one regression coefficient (e.g., byx) is strictly greater than 1, then for their product to remain less than or equal to 1, the other regression coefficient (bxy) 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.

Since Assertion (A) is false and Reason (R) is true, the correct option is (d).
Q15
Assertion (A): The two regression coefficients and the coefficient of correlation have the same signs.

Reason (R): If one regression coefficient is less than one than the other regression coefficient is always greater than one.
Solution

Step 1: Analyze Assertion (A).
The signs of byx, bxy, and r 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: byx×bxy1.
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.

Assertion (A) is true but Reason (R) is false. The correct option is (c).

Subjective Questions

Q16
The data regarding marks in statistics and mathematics is tabulated below:
Marks in Statistics (X)2535323136344245
Marks in Mathematics (Y)4344494135303246
(a) Find the coefficient of correlation between marks in statistics and mathematics.
(b) Find the most likely marks in statistics when the marks in mathematics is 48.
Solution

Let X represent marks in Statistics and Y represent marks in Mathematics. Number of observations, N=8.

Step 1: Calculate the arithmetic means.
X=25+35+32+31+36+34+42+45=280
X¯=2808=35

Y=43+44+49+41+35+30+32+46=320
Y¯=3208=40

Step 2: Construct the calculation table.
Let x=X-X¯ and y=Y-Y¯.

XYx=X-35y=Y-40x2y2xy
2543-1031009-30
3544040160
3249-39981-27
3141-41161-4
36351-5125-5
3430-1-10110010
42327-84964-56
45461061003660
28032000276332-52

(a) Calculating the coefficient of correlation (r):
Using the formula for direct deviation method: r=xyx2×y2 r=-52276×332=-5291632-52302.71-0.1718

(b) Calculating the most likely marks in Statistics (X) when Mathematics (Y) is 48:
We need to formulate the regression equation of X on Y. First, find the regression coefficient bxy: bxy=xyy2=-52332-0.1566 The regression equation of X on Y is given by: X-X¯=bxy(Y-Y¯) Substitute the known values: X-35=-0.1566(Y-40) Now, to estimate X when Y=48: X-35=-0.1566(48-40) X-35=-0.1566(8) X-35=-1.2528 X=35-1.2528=33.7472

Note: If we answer exactly what is mathematically asked (predicting Statistics from Mathematics), the value is ~33.75. However, if the question intended to ask for the most likely marks in Mathematics (Y) when the marks in Statistics (X) is 48, we would compute the regression line of Y on X. In that case, byx=-52276-0.1884, giving Y-40=-0.1884(48-35)Y=37.55, which rounds to 37.56 (which matches the textbook answer key).
(a) Correlation coefficient r-0.17
(b) Most likely marks in Statistics when Mathematics is 48 is 33.75. (Or 37.56 if predicting Mathematics from Statistics).
Q17
The height (in cms) of mothers and daughters is given below:
Height of mothers (X)148149150152153155157160
Height of daughters (Y)120125122128130140133150
(a) Find the line of regression of height of mothers on height of daughters.
(b) Estimate the height of mother when the height of daughter is 145 cm.
Solution

Let X represent height of mothers and Y represent height of daughters. Number of observations, N=8.

Step 1: Calculate the arithmetic means.
X=148+149+150+152+153+155+157+160=1224
X¯=12248=153

Y=120+125+122+128+130+140+133+150=1048
Y¯=10488=131

Step 2: Construct the calculation table to find deviations.
Let x=X-X¯ and y=Y-Y¯.

XYx=X-153y=Y-131x2y2xy
148120-5-112512155
149125-4-6163624
150122-3-998127
152128-1-3193
1531300-1010
1551402948118
157133421648
16015071949361133
1224104800120694268

(a) Line of regression of mother (X) on daughter (Y):
First, find the regression coefficient bxy: bxy=xyy2=2686940.38616 The equation for the line of regression of X on Y is: X-X¯=bxy(Y-Y¯) X-153=0.386(Y-131)

(b) Estimating height of mother (X) when height of daughter (Y) is 145 cm:
Substitute Y=145 into the regression equation: X-153=0.386(145-131) X-153=0.386(14) X-153=5.404 X=153+5.404=158.404

(a) Equation: X-153=0.386(Y-131)
(b) Estimated height of mother is 158.4 cm.
Q18
The following table gives the height in inches (X) and the weight in Lb. (Y) of a random sample of 10 students from a large group of students.
X61625863646564676972
Y112115108118120125119125130140
Find the estimate of weight of the student whose height is 70 inches.
Solution

Let X represent height and Y represent weight. Number of observations, N=10.

Step 1: Calculate the arithmetic means.
X=61+62+58+63+64+65+64+67+69+72=645
X¯=64510=64.5

Y=112+115+108+118+120+125+119+125+130+140=1212
Y¯=121210=121.2

Step 2: Use Assumed Means to simplify calculations.
Let assumed mean for X be A=64 and for Y be B=120.
Define deviations: dx=X-64 and dy=Y-120.

XYdxdydx2dy2dx×dy
61112-3-896424
62115-2-542510
58108-6-123614472
63118-1-2142
6412000000
65125151255
641190-1010
671253592515
691305102510050
7214082064400160
6451212512149788338

Step 3: Calculate the regression coefficient of Y on X (byx):
Since we need to predict weight (Y) given height (X), we compute byx using the shortcut formula: byx=Ndxdy-(dx)(dy)Ndx2-(dx)2 byx=10(338)-(5)(12)10(149)-(5)2 byx=3380-601490-25=332014652.2662

Step 4: Formulate regression line Y on X and predict Y.
The equation is: Y-Y¯=byx(X-X¯) Y-121.2=2.2662(X-64.5) Substitute the requested height X=70 inches: Y=121.2+2.2662(70-64.5) Y=121.2+2.2662(5.5) Y=121.2+12.4641=133.6641

The estimated weight of the student is 133.7 Lbs.
Q19
Obtain the two lines of regression from the following data:
X¯=50, Y¯=40, Sx=2, Sy=3, and r=0.8
Solution

Step 1: Calculate the regression coefficients.
Using standard deviations, the regression coefficient of Y on X (byx) is: byx=rSySx=0.8×32=0.8×1.5=1.2 The regression coefficient of X on Y (bxy) is: bxy=rSxSy=0.8×23=1.630.533

Step 2: Form the regression equation of Y on X. Y-Y¯=byx(X-X¯) Y-40=1.2(X-50) Y-40=1.2X-60Y=1.2X-20

Step 3: Form the regression equation of X on Y. X-X¯=bxy(Y-Y¯) X-50=0.533(Y-40) X-50=0.533Y-21.32X=0.533Y+28.68

Line of Y on X: Y=1.2X-20
Line of X on Y: X=0.533Y+28.68
Q20
Given the following information regarding the amount of rainfall (in inches) and yield of rice in quintals per acre:
Rainfall (in inches) [X]Production (per acre) [Y]
Mean2540
Standard Deviation46
The coefficient of correlation between the amount of rainfall and the production yield is 0.9. What is the possible yield of crop when the rainfall is 36 inches?
Solution

Step 1: Identify the variables and knowns.
Let X be Rainfall and Y be Production Yield. We need to estimate Y given X=36. We must find the regression equation of Y on X.
X¯=25, σx=4
Y¯=40, σy=6
r=0.9

Step 2: Calculate byx. byx=rσyσx=0.9×64=0.9×1.5=1.35

Step 3: Construct the regression line and estimate Y. Y-Y¯=byx(X-X¯) Y-40=1.35(X-25) Substitute X=36: Y=40+1.35(36-25) Y=40+1.35(11) Y=40+14.85=54.85

The possible yield is 54.85 quintals per acre.
Q21
For 5 pairs of observations the following results are obtained:
X=15, Y=25, X2=55, Y2=135, XY=83.
Find the equation of the line of regression and estimate the value of X when Y = 12.
Solution

Step 1: Understand the objective.
We are asked to estimate X when Y is given. This requires the regression line of X on Y.

Step 2: Calculate the means.
Given N=5.
X¯=XN=155=3
Y¯=YN=255=5

Step 3: Calculate the regression coefficient bxy.
Using raw data formula: bxy=NXY-(X)(Y)NY2-(Y)2 bxy=5(83)-(15)(25)5(135)-(25)2 bxy=415-375675-625=4050=0.8

Step 4: Formulate the regression line and estimate X.
Equation of X on Y: X-X¯=bxy(Y-Y¯) X-3=0.8(Y-5) Substitute Y=12: X=3+0.8(12-5) X=3+0.8(7)=3+5.6=8.6

Equation: X-3=0.8(Y-5). Estimated value of X is 8.6.
Q22
Find the mean of X and Y and the coefficient of correlation between them from the following two regression lines:
2Y-X-50=0
3Y-2X-10=0
Solution

Step 1: Find the means by solving the equations simultaneously.
Since the regression lines intersect at (X¯,Y¯), we solve:
(1) X=2Y-50
(2) 2X=3Y-10
Substitute (1) into (2): 2(2Y-50)=3Y-10 4Y-100=3Y-10Y=90 Substitute Y=90 into (1): X=2(90)-50=180-50=130 So, X¯=130 and Y¯=90.

Step 2: Identify the regression equations to find correlation (r).
Assume 2Y-X-50=0 is the line of Y on X. Rearranging for Y: 2Y=X+50Y=0.5X+25 Thus, byx=0.5.
Assume 3Y-2X-10=0 is the line of X on Y. Rearranging for X: 2X=3Y-10X=1.5Y-5 Thus, bxy=1.5.
Check validity: r2=byx×bxy=0.5×1.5=0.75. Since 0.751, our assumption is correct.

Step 3: Calculate r. r=±0.75 Since both coefficients are positive, r must be positive: r=0.750.866

Mean of X = 130, Mean of Y = 90, and Coefficient of Correlation r0.866.
Q23
On analyzing some data the two equations of regression are given as 2x=8-3y and 2y=5-x.
(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.
Solution

(i) Arithmetic means:
Solve the system: (1) 2x+3y=8 and (2) x+2y=5x=5-2y.
Substitute x into (1): 2(5-2y)+3y=8 10-4y+3y=8y=2 So, x=5-2(2)=1. Therefore, x¯=1, y¯=2.

(ii) Identify regression equation of y on x:
Assume 2y=5-x is y on x. Then y=-0.5x+2.5, so byx=-0.5.
Assume 2x=8-3y is x on y. Then x=-1.5y+4, so bxy=-1.5.
r2=(-0.5)×(-1.5)=0.751. This is valid. The equation y on x is 2y=5-x.

(iii) Compute correlation coefficient:
r=-0.75-0.866 (Negative because coefficients are negative).

(iv) Find standard deviation of y:
Given Sx2=16Sx=4.
We know byx=rSySx. -0.5=-0.866(Sy4) Sy=4×0.50.866=20.8662.31 (Alternatively, using byxbxy=Sy2Sx2: -0.5-1.5=13Sy2=163Sy=432.31)

(i) x¯=1, y¯=2.
(ii) Equation y on x: 2y=5-x.
(iii) Correlation coefficient r-0.866.
(iv) Standard deviation of y is 2.31.
Q24
The two lines of regression are given by 8x+10y=25 and 16x+5y=12.
If the variance of x is 25, find the standard deviation of y.
Solution

Step 1: Identify the correct regression lines.
Assume 8x+10y=25 is the line of Y on X. Rearranging gives 10y=-8x+25y=-0.8x+2.5. So byx=-0.8.
Assume 16x+5y=12 is the line of X on Y. Rearranging gives 16x=-5y+12x=-516y+1216. So bxy=-0.3125.
Product r2=(-0.8)×(-0.3125)=0.251. The assumption is correct.

Step 2: Use the formula linking variances and regression coefficients.
We know: byxbxy=rSySxrSxSy=Sy2Sx2 Given Sx2=25: -0.8-0.3125=Sy225 2.56=Sy225 Sy2=2.56×25=64 Sy=64=8

The standard deviation of y is 8.

Case Study Analysis

Case Study
The American Express Company believes that its cardholders tend to travel more extensively than others. A comprehensive research was undertaken to determine the relationship between distance travelled (X, in thousands of miles) and charges (Y, in thousands of dollars).
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).
Solution

(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 byx=1.33 and means X¯=3.16, Y¯=4.28: Y-Y¯=byx(X-X¯) Y-4.28=1.33(X-3.16) Y=1.33X-4.2028+4.28Y=1.33X+0.08

(iii) Prediction:
Substitute X=6 (since X is measured in thousands of miles) into the equation: Y=1.33(6)+0.08 Y=7.98+0.08=8.06 Since Y is measured in thousands of dollars, the predicted charge is 8.06×1000.

(i) Positive and very high degree.
(ii) Equation: Y=1.33X+0.08.
(iii) Predicted charge is $8060.
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