pping method.</p>
    
    <h2 class="hasAnchor" id="references"><a class="anchor" href="#references"></a>References</h2>

    <p>Yang, S., Imbens G. W., Cui, Z., Faries, D. E., &amp; Kadziola, Z.
  (2016) Propensity Score Matching and Subclassification in Observational
  Studies with Multi-Level Treatments. Biometrics, 72, 1055-1065.
  <a href='https://doi.org/10.1111/biom.12505'>https://doi.org/10.1111/biom.12505</a></p>
<p>Abadie, A., &amp; Imbens, G. W. (2006). Large sample properties of matching
  esti