1. An image defect predictive diagnostic apparatus comprising: a processor programmed to acquire first characteristic values and second characteristic values from a plurality of information sources including one or more sensors in an image forming apparatus that forms an image by using an electrophotographic process, the first characteristic values including at least one of (i) toner concentrations and (ii) developing potentials, the first characteristic values having an influence on developability, the second characteristic values being different from the first characteristic values and including at least one of (i) pixel densities, (ii) humidities, and (iii) number of image-formed sheets; predict the occurrence of a dot image which has no relationship with the image by applying a predetermined first occurrence probability in response to determining that a frequency of the first characteristic values is greater than a first determined threshold value, and applying a predetermined second occurrence probability in response to determining that a frequency of the second characteristic values is greater than a second determined threshold value, wherein the predetermined second occurrence probability is smaller than the predetermined first occurrence probability; and transmit a notification to the image forming apparatus indicating a likely occurrence of the dot image if a sum of the predetermined first occurrence probability and the predetermined second occurrence probability exceeds a threshold, such that a message indicating maintenance is required is displayed at the image forming apparatus.