An epidemiological study of prostate cancer among elderly men followed 1,400 men, 65-79 years of age, without evidence of prostate cancer over several years. Every two years the investigators examined the men for prostate cancer. The results were as follows: 12 new cases at the first evaluation, 11 new cases at the second evaluation, 23 new cases at the third evaluation, and 28 new cases at the fourth (final) evaluation. The investigators also noted that 35 of the men had withdrawn from the study at the third evaluation. Based on this information, calculate and interpret the incidence density rate of prostate cancer in this group.