Table 1.

Characteristics of the three representative cohorts (1, 2.1, 2.2)

Cohort	Period	Number of consecutive cases	Clinical significance
1	1978–1982	80	Diagnosis without sonography (“pre-sono-cohort”): primary clinical screening with secondary (“late”) radiological diagnosis
2.1	1994–1996	91	Diagnosis with sonography, including first effects of a nationwide sono-screening in Austria instituted in 1992 (“sono-screening-cohort”): routine (“early”) sonographic diagnosis within the first 6 weeks of life
2.2	2003–2005	91	Diagnosis with established nationwide sono-screening, but including the referral of external non-screened “immigration” cases for open reduction (“mixed-sono-screening-cohort”)