Acase-control study was performed to determine whether head injury was associated with an increased risk of brain tumors in children. two hundred cases with brain cancer were identified from the state cancer registry and 200 controls were recruited from the same neighborhoods where the cases lived. the mothers of the children completed a questionnaire that asked them to describe their child's past history of head injury. the investigators found that the mothers of the children with brain tumors reported a past head injury for 70 of the cases while a past history of head injury was reported in 30 of the controls. what type of bias was likely to have influenced the findings of this study? length bias recall bias surveillance, diagnostic, or referral bias interviewer/recorder bias