Item-total correlation as the cause for the underestimation of the alpha estimate for the reliability of the scale

Jari Metsamuuronen

Volume : V, Issue : XI, November - 2016

Item-total correlation as the cause for the underestimation of the alpha estimate for the reliability of the scale

Jari Metsamuuronen

Abstract :

By modifying the basic formulae of the alpha coefficient for estimating the scale reliability, a simple coefficient is derived where the item-total correlation is seen. While knowing that the alpha coefficient gives the lower bound for the reliability except the case of (essential) tau-equivalency, the reason for the underestimation is in the process of calculating the item-total correlation. The article shows why and how much the item-total correlation underestimates the item discrimination in the deterministically discriminating dataset; even in the optimally constructed dataset, the underestimation may be 13%