As we noted, sample correlation coefficients range from -1 to +1. In practice, meaningful correlations (., correlations that are clinically or practically important) can be as small as (or -) for positive (or negative) associations. There are also statistical tests to determine whether an observed correlation is statistically significant or not (., statistically significantly different from zero). Procedures to test whether an observed sample correlation is suggestive of a statistically significant correlation are described in detail in Kleinbaum, Kupper and Muller. 1
The tetrachoric correlation coefficient assumes that the variable underlying each dichotomous measure is normally distributed.  The tetrachoric correlation coefficient provides "a convenient measure of [the Pearson product-moment] correlation when graduated measurements have been reduced to two categories."  The tetrachoric correlation should not be confused with the Pearson product-moment correlation coefficient computed by assigning, say, values 0 and 1 to represent the two levels of each variable (which is mathematically equivalent to the phi coefficient). An extension of the tetrachoric correlation to tables involving variables with more than two levels is the polychoric correlation coefficient.