title: interpolating in R yearly time series data with quarterly values
body: <p>I have a data set that has a list of IDs, year, and income. I am trying to interpolate the yearly values to quarterly values. </p>
  
  <pre><code>id = c(2, 2, 2, 3, 3, 3,4,4,4,5,5)
  year = c(2000, 2001, 2002, 2000,2001,2002, 2000,2001,2002,2000,2002)
  income = c(20, 24, 26, 30,34,36, 40,46,48,53,56)
  df = data.frame(id, year, income)
  </code></pre>
  
  <p>For e.g., I am looking to get the values of (interpolated) income for year-quarter 2000Q1, 2000Q2, 2000Q3, 2000Q4, 2001Q1, ... , 2001Q4. So the dataframe would be id,year-quarter, income. The income would be based on interpolated income. </p>
  
  <p>I realize when linear interpolating, the trend must only be based on the respective IDs. Any suggestions on how I would do the interpolation in R?</p>