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Table 1 Simple regression and multivariate first-order autoregressive models on the number of patients enrolled every quarter.

From: Determinants of patient recruitment in a multicenter clinical trials group: trends, seasonality and the effect of large studies

   All ACTG studies Excluding substudies
   Univariate Multivariate* Univariate Multivariate
Parameters   Effect (95% CI) Effect (95% CI) Effect (95% CI) Effect (95% CI)
Time trend effect (per quarter)   27.1 (18.9, 35.4) ¶ 24.5 (17.6, 31.5) ¶ 16.1(10.1, 22.1) ¶ 14.7 (9.4, 20.2) ¶
Large studies effect ‡ (per point weight) 607.9 (284.5, 931.4) ¶ 701.0 (507.7, 894.4) ¶ 403.0 (185.6, 20.3) ¶ 415.1 (261.0, 569.3) ¶
Early starting effect §   -1196.0 (-1858.4,-533.7) ¶ -141.6 (-577.6, 294.3) -947.4 (-1367.8, -527.0) ¶ -429.2 (-786.9, -47.9) #
Studies starting in same quarter (per study) 59.6 (25.2, 94.0) ¶ 31.2 (10.6, 51.8) # 0.7 (-35.9,37.2) 1.6 (-22.0,25.3)
Seasonal effect      
  Spring 277.7 (-267.4, 722.8) 126.3 (-89.5, 342.1) 166.9 (-164.4, 498.1) 147.7 (-44.2, 339.5)
  Summer 136.9 (-358.3, 632.0) 115.5 (-110.4, 341.3) 75.5 (-255.7, 406.7) 41.9 (-150.2, 233.9)
  Fall 31.0 (-455.2, 517.2) 27.8 (-181.7, 237.2) 21.4 (-303.9, 346.6) 41.8 (-141.5, 225.1)
AR1    0.15 (-0.14, 0.45)   0.06 (-0.24, 0.35)
  1. ACTG: AIDS Clinical Trials Group, CI: confidence interval AR1: first-order autoregressive parameter * Constant in model = 109.7 (p = 0.47) † Constant in model = 520.3 (p < 0.001) ‡ Sum of weights for large studies in each quarter: ½ for a large study which is initiated in that quarter, 1 for a large study which is on its second quarter of enrollment, and ½ for a large study which is on its third quarter of enrollment. §Dummy variable for first 3 quarters (Fall 1986-Spring 1987) ¶ p < 0.001 # 0.001 ≤ p < 0.05