Characteristics of hospital nursing staff and patient outcomes: a commentary on Twigg et al (2010).

The study of the effect of hospital nurse staffing characteristics on patient morbidity and mortality by Twigg et al. (2010) hasmany strengths, including very high sample size and sophisticated statistical analysis. However, due to weaknesses that are not fully addressed in the study, the principal conclusion is not at all supported by the study findings. The conclusions are also self-contradictory, as explained below. First, the study concluded that the findings provide evidence that supports the NHPPD staffing method. The NHPPD staffing method was not studied though; the sum of the nursing hours was studied irrespective of whether the nursing hours had the pattern of the NHPPD formula. Consequently, the only conclusion that can be drawn from the findings is that increased nursing hours are associated with improved patient outcomes, and not the NHPPD formula. To determine the conclusion above, hospitals that increase nursing levels by a formula other than NHPPD would need to be compared with those that increase it by NHPPD, and of course, this was not the structure of this study. Second, the study concluded that this evidence must influence policy regarding nurse staffing levels. This is not consistent with their statement that the findings only provide evidence or add evidence that supports NHPPD staffing levels. In other words, it appears that the assumption is made that if you find any evidence for something, then it must be completely and automatically true. Last, the study used a non-experimental design called a time series design rather than an experimental method, and thus suffers from a lack of internal validity, as do all studies of this design. Without a control group or manipulated variable, cause-and-effect cannot be determined nor assumed. Specifically, the authors cannot conclude that the increased nurse staffing levels were the cause of the improved patientmortality andmorbidity. This is readily explained with an example. Suppose that a hospital has a lower patientmortality in 2009 than in 2008. If that hospital also put up more pictures of flowers in the hallways of the hospital in 2009 than in 2008, a statistical analysis of the type used in this study would find a strong association between the number of pictures and the death rate. If the same logic was used as in the above study, the conclusion would be that a policy must be established to require more flower pictures in hospitals everywhere. Although this argument seems absurd when expressed in this manner, in that it seems self-evident that increased nursing hours should be beneficial, if it was self-evident, studies would not need to be done on this question. This study is actually one case of a widespread problem in the hospital research, most of which uses non experimental designs. Improvement in patient outcomes have been attributed to multidisciplinary communication (Vazirani et al., 2005), use of medical emergency teams (Bellomo et al., 2003), type and frequency of rounds (Dutton et al., 2003), early identification and treatment of International Journal of Nursing Studies 48 (2011) 655–656