Pitprops Strength Correlation Data


This correlation matrix was published in Jeffers (1967) and was calculated from 180 observations. The 13 variables were used as explanatory variables in a regression problem which arised from a study on the strength of pitprops cut from home-grown timber.




Its a correlation matrix of 13 variables which have the following meaning:

[,1] TOPDIAM Top diameter of the prop in inches
[,2] LENGTH Length of the prop in inches
[,3] MOIST Moisture content of the prop, expressed as a percentage of the dry weight
[,4] TESTSG Specific gravity of the timber at the time of the test
[,5] OVENSG Oven-dry specific gravity of the timber
[,6] RINGTOP Number of annual rings at the top of the prop
[,7] RINGBUT Number of annual rings at the base of the prop
[,8] BOWMAX Maximum bow in inches
[,9] BOWDIST Distance of the point of maximum bow from the top of the prop in inches
[,10] WHORLS Number of knot whorls
[,11] CLEAR Length of clear prop from the top of the prop in inches
[,12] KNOTS Average number of knots per whorl
[,13] DIAKNOT Average diameter of the knots in inches


Jeffers (1967) replaced these 13 variables by their first six principal components. As noted by Vines (2000), this is an example where simple structure has proven difficult to detect in the past.


Jeffers, J.N.R. (1967) Two case studies in the application of principal components analysis. Appl. Statist. 16, 225–236.

Vines, S.K. (2000) Simple principal components. Appl. Statist. 49, 441–451.



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