Correlation between the numbers of acid-fast bacilli visible by sputum smear-microscopy and the bleach-sedimentation concentrating effect. Each data point represents data from a single specimen. The horizontal axis shows the geometric mean number of acid-fast bacilli per 100 microscopy fields visualised in triplicate conventional smears. The horizontal error bars represent the standard error of the mean (SEM) for these triplicate data. The vertical axis shows the mean change in the number of acid-fast bacilli per 100 microscopy fields visualised in triplicate slides prepared after bleach-sedimentation of that specimen. The vertical error bars represent the SEM for these triplicate data. The diagonal broken line is the regression line. r represents the correlation coefficient. The box parallel with the horizontal axis indicates the smear-microscopy grade equivalent to the acid-fast bacilli counts per 100 microscopy fields (0 indicates none visible/100 fields; +/- indicates 1-9/100 fields; + indicates 10-99/100 fields; ++ indicates 100-999/100 fields; and +++ indicates > 1000/100 fields).