**Correction to: BMC Nursing**

**https://doi.org/10.1186/s12912-019-0333-3**

In the original publication of this article \[[@CR1]\], a formula's format in the section of "Sample size calculation" was wrong and needs to be revised.

The correct formula should be:$$\documentclass[12pt]{minimal}
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                \begin{document}$$ \mathbf{N}={\left[{\mathbf{Z}}_{\left(\mathbf{1}-\boldsymbol{\upalpha} \right)}\sqrt{\mathbf{2}\mathbf{P}\left(\mathbf{1}-\mathbf{P}\right)}+{\mathbf{Z}}_{\left(\mathbf{1}-\boldsymbol{\upbeta} \right)}\sqrt{\mathbf{P1}\left(\mathbf{1}-\mathbf{P}\mathbf{1}\right)+\mathbf{P}\mathbf{2}\left(\mathbf{1}-\mathbf{P}\mathbf{2}\right)}\right]}^{\mathbf{2}}/{\left(\mathbf{P}\mathbf{1}-\mathbf{P}\mathbf{2}\right)}^{\mathbf{2}}. $$\end{document}$$
