Dictionary of Evidence-based Medicine.pdf

Dictionary of Evidence-based Medicine 93
example in expressing the logistic model in a linear form. Note that
the logit model is a special case of the log linear model where the
y variable is the odds and the link function is the log of an odds (see under
Odds ratio, Generalized linear model, Logistic regression). (Bishop YM,
Fienberg SE, Holland PW (1975) Discrete multivariate analysis: theory and
practice. MIT Press, Cambridge, MA).
Log linear model
The log linear model is a special class of generalized linear models where
the link function takes the form:
The a and P values are the coefficients to be estimated in such models.
Note that y can take only positive values in such models (Agresti A (1996)
An introduction to categorical data analysis. John Wiley, New York).
Longitudinal study (see a/so under Cohort study)
A study in which subjects are recruited and followed up forward in time,
usually over many months or years. For example, in such a study, diabetic
patients may be followed over a period of years to investigate how good
blood glucose control affects the likelihood of complications, such as
cataracts or peripheral neuropathy, developing.
Lorenz curve
The Lorenz curve is a useful graphical method, invented by a 19th century
German statistician for depicting the distribution of income in a population.
If one were to plot the cumulative proportion of total income versus
the cumulative proportion of income recipients in a population, we would
observe a straight line with unit slope, given perfect equality in income
among individuals (Figure 14). The line will curve downwards if the income
distribution is unequal and the more unequal the distribution, the
further the curve will be from the diagonal (Brown WS (1995) Principles
of economics. West Publishing Company, Minneapolis). The Lorenz curve
can be used to calculate a coefficient of inequality, referred to as the Gini
coefficient. This is the ratio of the area between the line of equality and the
Lorenz curve (area A) and the area below the line of equality (areas A + B).
The Gini coefficient has a useful quantitative economic interpretation. A
value of 0.25 can be interpreted as meaning that if two incomes are drawn