There are two aspects of interest regarding how Web users' interact with the images that they download in Web pages: the users' perceptual limits, and their tolerance to visibly degraded images under differing conditions and in different contexts. Establishing the mean JPD over a number of images not only allows the calculation of a cut-off point for default policies, but also delimits the variation that can meaningfully offered to users for customisation of their policy.
The hypothesis proposed that the mean of the sample (25.42) plus two times the standard deviation (8.48) giving a further compression of around 57% would be practicable. An alternative would be to adopt the mean plus one standard deviation, a value of 25.42. This would also be meaningful - 16% of users would notice only slight degradation in image quality and the compression would be higher at 78%. In other words, a meaningful cut-off point does not emerge from the statistics but must be chosen.
The probabilities of similarity, derived from performing a pairwise T-Test between each of the pictures, are very revealing. There are highly significant differences between each of the images - except between the face and the map.
Authors should be made aware of these findings. In particular, their tools should be designed to encourage the use of a default setting that reflects the cut-off point established here. Images intended for use in web pages generated by popular graphic and art packages can be saved at a JPEG index of 34. Finally, authors should be encouraged to write policies in which the quality of the image reflects its importance.
In conclusion, the variation in subjects' scores combined with the
differences across images encourages a conservative estimate of
s+2 
SD. However,if only s+SD is used, a few may notice the degradation
but it is a small price to pay for the added compression. The cut-off
point of s+SD (JPEG index of 34) will be used for the systems default
policy.