Multiple imputation for unit-nonresponse versus weighting including a comparison with a nonresponse follow-up study

The results of a national fear of crime survey are compared with results following the use of different nonresponse correction procedures. We compared naive estimates, weighted estimates, estimates after a thorough nonresponse follow-up and estimates after multiple imputation. A strong similarity between the MI and the follow-up-estimates was found. This suggests, that if the assumptions of MAR hold, carefully selected and collected additional data applied in a MI could yield similar estimates to a nonresponse follow-up at a much lower price and respondent burden. --Multiple Imputation,Unit-nonresponse,missing data,complex surveys.

Multiple imputation for unit-nonresponse versus weighting including a comparison with a nonresponse follow-up study

"The results of a national fear of crime survey are compared with results following the use of different nonresponse correction procedures. We compared naive estimates, weighted estimates, estimates after a thorough nonresponse follow-up and estimates after multiple imputation. A strong similarity between the MI and the follow-up-estimates was found. This suggests, that if the assumptions of MAR hold, carefully selected and collected additional data applied in a MI could yield similar estimates to a nonresponse follow-up at a much lower price and respondent burden." (author's abstract

Untersuchungen zur Bedeutung von GPR40 für die Generation Fettsäure-induzierter Ca2+-Signale und die Fettsäure-stimulierte Insulinsekretion in pankreatischen [beta]-Zellen

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Comparison of Improved Unidirectional Dual Velocity-Encoding MRI Methods

Background: In phase-contrast (PC) MRI, several dual velocity encoding methods have been proposed to robustly increase velocity-to-noise ratio (VNR), including a standard dual-VENC (SDV), an optimal dual-VENC (ODV), and bi- and triconditional methods. Purpose: To develop a correction method for the ODV approach and to perform a comparison between methods. Study Type: Case–control study. Population: Twenty-six volunteers. Field Strength/Sequence: 1.5 T phase-contrast MRI with VENCs of 50, 75, and 150 cm/second. Assessment: Since we acquired single-VENC protocols, we used the background phase from high-VENC (VENCH) to reconstruct the low-VENC (VENCL) phase. We implemented and compared the unwrapping methods for different noise levels and also developed a correction of the ODV method. Statistical Tests: Shapiro–Wilk's normality test, two-way analysis of variance with homogeneity of variances was performed using Levene's test, and the significance level was adjusted by Tukey's multiple post hoc analysis with Bonferroni (P &lt; 0.05). Results: Statistical analysis revealed no extreme outliers, normally distributed residuals, and homogeneous variances. We found statistically significant interaction between noise levels and the unwrapping methods. This implies that the number of non-unwrapped pixels increased with the noise level. We found that for β = VENCL/VENCH = 1/2, unwrapping methods were more robust to noise. The post hoc test showed a significant difference between the ODV corrected and the other methods, offering the best results regarding the number of unwrapped pixels. Data Conclusions: All methods performed similarly without noise, but the ODV corrected method was more robust to noise at the price of a higher computational time. Level of Evidence: 4. Technical Efficacy Stage: 1.</p