– 35 – Table 4. Mid-IR Variability of Photometrically-Selected AGNs. Variability Variability MIPS QSOs X-ray FIRST WSRT Level σ$_{12}$ Number (fraction/false positive rate) of variable objects: Photometry Group 1 Total ... 1964 3231 313 2046 2 > 2 230 (11.7%/5%) 392 (12.1%/4%) 12 (3.8%/8%) 48 (2.3%/13%) 3 > 3 151 (7.7%/3%) 253 (7.8%/2%) 8 (2.6%/3%) 27 (1.3%/8%) 4 > 4 107 (5.4%/2%) 164 (5.1%/1%) 4 (1.3%/1%) 17 (0.8%/4%) Number (fraction/false positive rate) of variable objects: Photometry Group 2 Total ... 1942 2950 292 1883 2 > 2 229 (11.8%/5%) 391 (13.3%/3%) 12 (4.1%/7%) 48 (2.5%/10%) 3 > 3 151 (7.8%/3%) 253 (8.6%/2%) 8 (2.7%/3%) 27 (1.4%/6%) 4 > 4 107 (5.5%/2%) 164 (5.6%/1%) 4 (1.4%/1%) 17 (0.9%/3%) Number (fraction/false positive rate) of variable objects: Photometry Group 3 Total ... 1637 2217 195 1233 2 > 2 225 (13.7%/4%) 374 (16.9%/2%) 11 (5.6%/4%) 43 (3.5%/9%) 3 > 3 151 (9.2%/3%) 245 (11.1%/1%) 8 (4.1%/1%) 25 (2.0%/5%) 4 > 4 107 (6.5%/1%) 160 (7.2%/1%) 4 (2.1%/1%) 15 (1.2%/4%) Note. — The fraction of variable sources is a ratio of the number of variable objects above a given level of σ$_{12}$ and r > 0 . 8 to the total number of objects in a respective class. The false positive rate is estimated from the number of sources above the same level of σ$_{12}$ but with r < 0 . 5 (see Section 3 and Figure 3).