title: Single-case experimental design yielded an effect estimate corresponding to a randomized controlled trial.
abstract: We reanalyzed data from a previous randomized crossover design that administered high or low doses of intravenous immunoglobulin (IgG) to 12 patients with hypogammaglobulinaemia over 12 time points, with crossover after time 6. The objective was to see if results corresponded when analyzed as a set of single-case experimental designs vs. as a usual randomized controlled trial (RCT).
  
  
  
  Two blinded statisticians independently analyzed results. One analyzed the RCT comparing mean outcomes of group A (high dose IgG) to group B (low dose IgG) at the usual trial end point (time 6 in this case). The other analyzed all 12 time points for the group B patients as six single-case experimental designs analyzed together in a Bayesian nonlinear framework.
  
  
  
  In the randomized trial, group A [M = 794.93; standard deviation (SD) = 90.48] had significantly higher serum IgG levels at time six than group B (M = 283.89; SD = 71.10) (t = 10.88; df = 10; P < 0.001), yielding a mean difference of MD = 511.05 [standard error (SE) = 46.98]. For the single-case experimental designs, the effect from an intrinsically nonlinear regression was also significant and comparable in size with overlapping confidence intervals: MD = 495.00, SE = 54.41, and t = 495.00/54.41 = 9.10. Subsequent exploratory analyses indicated that how trend was modeled made a difference to these conclusions.
  
  
  
  The results of single-case experimental designs accurately approximated results from an RCT, although more work is needed to understand the conditions under which this holds.
authors.affiliation: Department of Psychological Sciences, University of California Merced, 5200 N Lake Rd., Merced, CA 95343, USA.
authors.forename: William R
authors.initials: WR
authors.lastname: Shadish
authors.affiliation: Educational Psychology, CUNY Graduate Center, New York, NY 10036-8003, USA. Electronic address: drindskopf@gc.cuny.edu.
authors.forename: David M
authors.initials: DM
authors.lastname: Rindskopf
authors.affiliation: Public Health Group, School of Social Sciences, Humanities, and Arts, University of California Merced, Merced, CA 95343, USA.
authors.forename: Jonathan G
authors.initials: JG
authors.lastname: Boyajian
mesh_terms: D061605:Administration, Intravenous; D000361:Agammaglobulinemia / Q000188:drug therapy*; D001499:Bayes Theorem; D035843:Biomedical Research / Q000379:methods*; D004305:Dose-Response Relationship, Drug; D006801:Humans; D007136:Immunoglobulins / Q000008:administration & dosage*; D016032:Randomized Controlled Trials as Topic; D012107:Research Design; D013223:Statistics as Topic / Q000379:methods*; D013997:Time Factors
chemical_list: D007136:Immunoglobulins
keywords: Comparative effectiveness; Evaluation; Randomized controlled trial; Research methods; Single-case experimental designs; Within-study comparison