Equity Estimation and Assessing Market Response

Equity estimation has been proposed as a possibly superior technique for estimating market response functions in the presence of high predictor-variable collinearity. The relative performance of equity, ridge, and ordinary least squares (OLS) estimators is examined using simulation experiments. In part, findings are consistent with prior research and indicate that, under certain conditions, equity outperforms OLS and ridge on a number of important criteria, and equity yields coefficient estimates that assign more equal explanatory weight to correlated predictor variables than does OLS or ridge. As collinearity increases, this tendency becomes very pronounced, to the point where equity yields estimated standardized coefficients more equal in magnitude irrespective of other conditions, such as true coefficient values and model explanatory power.

Influence of Variability of Interpretation of Contrast Venography for Screening of Postoperative Deep Venous Thrombosis on the Results of a Thromboprophylactic Study

SummaryTo assess interobserver variability of venography for screening for postoperative deep venous thrombosis (DVT), we used 185 bilateral ascending contrast venograms (366 lower limbs) which were performed using the long-leg film technique, in the frame of a trial of the efficacy of two low-molecular-weight-heparin fractions (initial evaluation). These venograms were submitted in a multicenter setting to three further readers who performed a serial scoring as DVT, no DVT or non-evaluable. DVTs were diagnosed in 78, 55 and 59/366 limbs (initial evaluation: 58). The proximal locations of DVTs were 16, 9 and 16 (initial evaluation: 15) and the non evaluable limbs 3, 5 and 18 (initial evaluation: 0). Apparent pairwise agreement between the three readers ranged from 87 to 90% (true coefficient of agreement Kappa 0.63-0.70). It ranged from 89 to 93% (Kappa 0.63-0.74) for the comparison between the readers and the initial evaluation. This considerable degree of disagreement among experienced readers should be taken into account in calculating sample sizes in prospective thromboprophylactic studies. Indeed, one of the two prophylactic regimens proved to be significantly (p = 0.012, p = 0.031, p = 0.049) or non-significantly (p = 0.073) superior to the other one depending upon the reading of venograms.