title: How to subset all rows measured using different measurement tools from a data frame for the longitudinal study?
body: <p>I have a data set consisting of repeated measures(4 waves), one data point per row. These data are from a longitudinal study, and measured two different tools based on the study subjects age; therefore, at the moment, not every ID has assessed using one measuring tool at wave one.
  In the data
  subject 1,4,7 has four repeated measures BITSEA at wave 1 and 2,3 and 4 CBCL
  subject 5 &amp; 7 have four repeated measures CBCL at all waves</p>
  
  <p>I want to be able to extract data points involved in all repeated measures. I wish to obtain those with four repeated measures using BITSEA if subjects are between 9 and 35 months else CBCL at wave 1.
  The background of my question is to create a data frame for those who have measured in all waves subsetting subjects measured at wave 1 using either BITSEA or CBCL.
  Any thoughts, please?
  the data looks like</p>
  
  <pre><code>dat &lt;- read.table(structure(list(subject_id = c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 3L, 3L, 4L, 4L, 4L, 4L, 5L, 5L, 5L, 5L,6L, 6L, 7L, 7L, 7L, 7L, 8L, 8L, 8L, 9L, 10L, 10L, 10L, 10L), wave = c("wave1", "wave2", "wave3", "wave4", "wave1", "wave2", "wave3", "wave1", "wave2", "wave1","wave2", "wave3", "wave4", "wave1", "wave2", "wave3", "wave4","wave1", "wave2", "wave1", "wave2", "wave3", "wave4", "wave1", "wave2", "wave3", "wave1", "wave1", "wave2", "wave3", "wave4"), AgeinMonths = c(10L, 38L, 56L, 76L, 38L, 46L, 66L, 23L, 41L,35L, 56L, 76L, 98L, 36L,54L, 74L, 92L, 38L, 58L, 11L, 36L, 49L, 69L, 17L, 37L, 55L, 36L, 36L, 54L, 72L, 90L), BITSEA = c("7","Not applicable", "Not applicable", "Not applicable", "Not applicable", "Not applicable", "Not applicable", "27", "Not applicable", "28", "Not applicable", "Not applicable", "Not applicable", "Not applicable", "Not applicable", "Not applicable", "Not applicable", "Not applicable","Not applicable", "23", "Not applicable", "Not applicable", "Not applicable", 
           "26", "Not applicable", "Not applicable", "Not applicable", "Not applicable", "Not applicable", "Not applicable", "Not applicable"),                                CBCL = c(NA, "66", "73", "87", "57", "63", "77", NA, "38", NA, "70", "77",  "97", "66", "45", "67", "68", "54", "63","Not applicable", "45",  "51", "65", "Not applicable", "45", "57", "56", "45", "55", "65,  "71"))
  </code></pre>