“…A number of prospective studies have reported data that make it more challenging to determine if regression has occurred. These include studies in which only standardized scores [Estes et al, ; Landa, Gross, Stuart, & Bauman, ; Levin, Varcin, O'Leary, Tager‐Flusberg, & Nelson, Lord, Luyster, Guthrie, & Pickles, ] or group data are reported [Barbaro & Dissanayake, ;Caravella & Roberts, ; Chenausky, Nelson, & Tager‐Flusberg, ; Filliter et al, ; Gliga, Smith, Likely, Charman, & Johnson, ; Ibañez, Grantz, & Messinger, ; Iverson et al, ], where loss and plateau are not differentiated [Brian et al, ], or where longitudinal analysis was not carried out [Bedford, et al, ; Gangi et al, ]. Although these studies show progressive divergence from typical development, they cannot differentiate developmental arrest or slowing from regression, and group data may have obscured variability within groups [Elsabbagh & Johnson, ].…”