title: (Cubic) spline interpolation
body: <p>I want to perform a (cubic) spline interpolation for population data to &quot;transform&quot; yearly data into quarterly data. I know that there are a fair number of flaws doing so, but I need to do it.</p>
  <p>Here is an example of my code (using generic input data):</p>
  <pre><code>#--------------spline interpolation
  
  x &lt;- c(1973:2014)
  population &lt;- seq(500000, 600000, length.out=42)
  list &lt;- spline(x, population, n=4*length(x), method=&quot;fmm&quot;,
                xmin=min(x), xmax=max(x), ties=mean)
  
  x_spline &lt;- list$x
  pop_spline &lt;- list$y
  </code></pre>
  <p>How can I define that the splines are calculated &quot;quarterly&quot;, in other words at 1973.25, 1973.5, 1973.75, 1974 etc.? Sorry for not being an expert in statistics: What would be the best method to &quot;transform&quot; yearly data into quarterly data: &quot;fmm&quot;, &quot;natural&quot;, &quot;periodic&quot;, &quot;monoH.FC&quot; or &quot;hyman&quot;? The assumption would be that the growth of population is evenly distributed over the year.</p>