Inter-observer agreement on the interactive threshold technique. (A) Scatter plot illustrating the inter-observer correlation (Reader 1 x-axis, Reader 2 y-axis) of the percentage mammographic density measures (PMD) by the interactive threshold technique based on 765 breasts*. The black diagonal line indicates perfect agreement between the two readers. The red dashed line is the line of best fit. (B) Bland-Altman plot illustrating inter-observer agreement. Difference in PMD measures (Reader 2 minus Reader 1) is plotted against the mean PMD. The blue line shows a bias of 0.009 (≈1%) indicating only slightly higher PMD measures by R2 on average. The upper (UAL) and lower (LAL) 95% agreement limits are illustrated by the red dashed lines. *Each PMD measure is an average of the CC and MLO value. Only the MLO view was available in 8 breasts. These have been included with a corrected value after linear regression analysis.