Chapter 12.4, Problem 56E

The article “Bone Density and Insertion Torque as Predictors of Anterior Cruciate Ligament Graft Fixation Strength” (The Amer. J. of Sports Med., 2004: 1421–1429) gave the accompanying data on maximum insertion torque (N · m) and yield load (N), the latter being one measure of graft strength, for 15 different specimens.

Torque	1.8	2.2	1.9	1.3	2.1	2.2	1.6	2.1
Load	491	477	598	361	605	671	466	431

Torque	1.2	1.8	2.6	2.5	2.5	1.7	1.6
Load	384	422	[554	577	642	348	446

1. a. Is it plausible that yield load is normally distributed?
2. b. Estimate true average yield load by calculating a confidence interval with a confidence level of 95%, and interpret the interval.
3. c. Here is output from Minitab for the regression of yield load on torque. Does the simple linear regression model specify a useful relationship between the variables?

Predictor	Coef	SE Coef	T	P
Constant	152.44	91.17	1.67	0.118
Torque	178.23	45 .97	3.88	0.002

Source	DF	SS	MS	F	P
Regression	1	80554	80554	15.03	0. 002
Residual Error	13	69684	5360
Total	14	150238

1. d. The authors of the cited paper state, “Consequently, we cannot but conclude that simple regression analysis-based methods are not clinically sufficient to predict individual fixation strength.” Do you agree? [Hint: Consider predicting yield load when torque is 2.0.]