Research Question and Hypotheses

Example

Selction Bias Exposure and Outcome -Misclassification
Author (Year) Research Question and Hypotheses Study Design Selection of Subjects Response Rate Length of Follow-up Exposure Outcome (disease) Confounding Statistical Analysis Results Rigor Score
1Research Question and Hypotheses Parr CL (2018) Is vitamin A and D intake during pregnancy and in early life associated with risk of asthma development? Vitamin A in urtero and supplemented during infancy will increase the risk of developing astma. Vitamin D in utero and supplemented during infancy will decrease the risk of developing asthma. Prospective Cohort Study allowing the exposure to be observed prior to the development of the disease. Women recruited at 18 weeks gestation as part of the Norwegian Mother and Child Cohort Study (N=115,398). Children linked to medical birth registry and Norweigian Prescription Database. 61,676 children included in the analysis. 41% to the Norwegian Mother and Child Cohort study- no other information 7 years Vitamin A and D estimated from a validated food frequency questionnaire which inquired about foods and supplements consumed since getting pregnant. Child infant supplements of A and D measured with a follow-up questionnaire inquiring about none, sometimes or daily supplementation. Childhood asthma evaluated at 7 years of age and defined as having 2 or more pharmacy dispensations of asthma medication within a 12 month interval (the first dispensed between ages 6-7 years). Potential confouders included: maternal age at delivery, parity, region of delivery, mode of delivery,child’s sex, birthweight, and gestational age, vitamin E, long-chain n-3 fatty acids, vitamin C, folate, total energy, leisure time pysical activity, solarium use in pregnancy, geographic region of delivery within Norway, season of delivery. maternal history of asthma or allergic disorders, breastfeeding, respiratory tract infection, maternal smoking, use of paracetamol or acetaminophen and use of antibiotics in first 6 months. Used relative risk and 95% confidence intervals. Presented both crude and adjusted estimates of effect. Maternal nutrients (Vit A and D) were analyzed as quintiles and compared all levels of intake to the lowest intake. Authors found that maternal inatke of vitamin A during pregnancy increased the risk of asthma development in offspring. Q1: RR=1.0; Q2 RR= 0.92 95% CI (0.80, 1.05); Q3 RR=0.99 95% CI (0.86,1.13); Q4 RR=1.08 95% CI(0.93,1.24); Q5 RR=1.21 95% CI(1.05,1.40). Conversely, vitamin D intake during pregnancy decreases the risk of asthma. Q1: RR=1.0; Q2 RR=0.90 95% CI (0.79,1.02); Q3 RR=0.89 95% CI (0.77, 1.03); Q4 RR=0.96 95% CI (0.82, 1.12); Q5 RR=0.81 95% CI (0.67,0.97). Daily infant supplementation with vitamin D was moderately protective RR=0.97 95% CI (0.86,1.09). Although finding demonstrate a dose response, most relative risks do not reach statistical significance. 70%. There is concern that criteria for the selection of study subjects was related to the FFQ and the ability of gain prescriptions for 23% of the MoBa cohort as well as a low response rate (41%) leaves concern for selection bias and the generalizability of findings. There is also some concern that vitamin A and D are misclassified if mom’s did not accurately report dietary inake since becoming pregnant.
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