Every two years , participants are mailed a questionnaire assessing risk factors and the interval occurrence of disease . Records of diagnoses of pulmonary embolism have been collected prospectively on each biennial questionnaire since study inception . Data describing time sitting were collected in 1988 and 1990 , and data describing physical activity have been collected since 1980 . We therefore began prospective follow - up in 1990 among women who responded to both the 1988 and 1990 questionnaires ( n=99 290 ) . Outcome ascertainment occurred from 1990 to 2008 .