title: Inducing a blind spot: blinding data collectors in an investigation of experimental pain.
abstract: OBJECTIVE
  Methods of blinding have been infrequently investigated. This study assessed data collector blinding within an investigation of endogenous pain modulation.
  
  
  METHODS
  Participants (N = 33; 52% women) were randomly assigned to an order of an exercise bout with intramuscular control injections, quiet rest with intramuscular algesic injections, and a control condition. Data collectors (N = 4; 50% women) recorded participants' pain responses and then left the room before the injection and exercise procedures were administered by other personnel. Immediately after the procedures, the data collectors returned to collect additional pain responses and record their suspicions regarding the assigned condition, their confidence level in their suspicions (0-100 mm scale), and the reason/s for their suspicions.
  
  
  RESULTS
  Data collectors correctly identified control and algesic injections in 90.4 and 73.1% of the sessions, respectively, and quiet rest and exercise in 94.2 and 69.2% of the sessions, respectively. The confidence of the data collectors in their suspicions was 60.17 mm (SD = 34.54) for injections and 62.19 mm (SD = 34.95) for exercise. However, data collectors only had correct suspicions and confidence of at least 90 mm for 26.2% of the injections and 29.4% of the exercise bouts. Participants' pain responses were the primary reason for data collectors' suspicions. Neither the amount of experience nor the sexual composition of the participant-data collector dyad influenced the blinding.
  
  
  CONCLUSIONS
  Collection of participants' pain responses led to frequent episodes of unblinding. However, it may be misleading to only consider the frequency of correct suspicions as successful or unsuccessful blinding.
authors: Department of Physical Therapy.
authors: Erin A
authors: EA
authors: Dannecker
authors: Department of Kinesiology, University of Wisconsin-Madison, Madison, Wisconsin, 53706, USA.
authors: Kelli F
authors: KF
authors: Koltyn
authors: Division of Rheumatology and Immunology, University of Missouri, Columbia, Missouri, 65211, USA.
authors: Celso R
authors: CR
authors: Velazquez
authors: Center for Sensory-Motor Interaction, Department of Health Science and Technology, Faculty Medicine, Aalborg University, Denmark.
authors: Lars
authors: L
authors: Arendt-Nielsen
authors: Center for Sensory-Motor Interaction, Department of Health Science and Technology, Faculty Medicine, Aalborg University, Denmark.
authors: Thomas
authors: T
authors: Graven-Nielsen
mesh_terms: D000293:Adolescent; D000328:Adult; D003625:Data Collection / Q000379:methods* / Q000592:standards*; D004311:Double-Blind Method; D005260:Female; D006801:Humans; D008297:Male; D010146:Pain / Q000175:diagnosis* / Q000523:psychology; D010147:Pain Measurement / Q000379:methods* / Q000523:psychology / Q000592:standards*; D017288:Pain Threshold / Q000502:physiology / Q000523:psychology; D016037:Single-Blind Method; D055815:Young Adult
keywords: Blinding; Experimental Pain; Research Personnel