title: Importance of using rigorous statistical methods to analyze low energy laser experimental data: Part two.
abstract: OBJECTIVE
  Numerous authors have reported successful alteration of peripheral nerve action potential characteristics through application of low energy laser irradiation (LELI). The statistical analysis that accompanies many of these reports frequently does not account for the special nature of the data generated in typical LELI experiments. The objective of this study was to evaluate the application of repeated measures linear regression techniques to the analysis of this type of data. Issues of analyzing raw versus normalized data, proper accounting for correlation between measurements, and discrete time point hypothesis testing were addressed.
  
  
  METHODS
  The data analyzed in this work were generated from an experiment in which in vitro frog sciatic nerves were irradiated with a helium-neon laser using a variety of treatment protocols. Compound action potential (CAP) amplitude, latency, depolarization rate, and repolarization rate were recorded at 1-minute intervals for 135 minutes for each nerve. Laser-induced changes in CAP parameters were analyzed using various repeated measures linear regression models.
  
  
  RESULTS
  The findings of statistical significance were highly dependent on the rigor of the regression model applied. Application of the same regression model to raw and normalized data produced different findings of significance. Determination of significant contrasts was highly dependent on how well the regression model accounted for the correlation between repeated measurements made on the same nerve. In general, models that failed to account adequately for this correlation produced more findings of significant contrasts than increasingly rigorous models. Finally, discrete time point hypothesis testing on normalized data can suggest improper statistical conclusions if the proper correlation structure is not applied to the data set.
  
  
  CONCLUSIONS
  Linear regression analysis offers advantages over discrete time point hypothesis testing in the analysis of highly correlated serial data of this type. Trends in the behavior of the measured parameters are evident, rigorous accounting for correlation between measurements is facilitated, and hypothesis testing is highly flexible.
authors.affiliation: Biomedical Engineering Center, Ohio State University, Columbus, USA.
authors.forename: D W
authors.initials: DW
authors.lastname: Ebert
authors.forename: C
authors.initials: C
authors.lastname: Roberts
mesh_terms: D000200:Action Potentials / Q000528:radiation effects; D000818:Animals; D066298:In Vitro Techniques; D007834:Lasers; D016014:Linear Models; D008959:Models, Neurological; D017933:Peripheral Nervous System / Q000502:physiology / Q000528:radiation effects; D011892:Rana catesbeiana; D012584:Sciatic Nerve / Q000502:physiology / Q000528:radiation effects*; D013997:Time Factors