title: Wrong result of mean with dplyr
body: <p>I am a beginner in R and I have a big data.frame (more than 300000 obs) that look like this:</p>
  
  <pre><code>Dados &lt;- data.frame(stringsAsFactors=FALSE,
                 id = c(1L, 2L, 3L, 4L, 5L, 6L, 7L, 8L, 9L, 10L, 11L, 12L, 13L,
                        14L, 15L, 16L, 17L, 18L, 19L, 20L, 21L, 22L, 23L, 24L,
                        25L, 26L, 27L, 28L, 29L, 30L, 31L, 32L, 33L, 34L, 35L, 36L,
                        37L, 38L, 39L, 40L, 41L, 42L, 43L, 44L, 45L, 46L, 47L, 48L,
                        49L, 50L, 51L, 52L, 53L, 54L, 55L, 56L, 57L, 58L, 59L, 60L,
                        61L, 62L, 63L, 64L, 65L, 66L, 67L, 68L, 69L, 70L, 71L,
                        72L, 73L, 74L, 75L, 76L, 77L, 78L, 79L, 80L, 81L, 82L, 83L,
                        84L, 85L, 86L, 87L, 88L, 89L, 90L, 91L, 92L, 93L, 94L, 95L,
                        96L, 97L, 98L, 99L, 100L, 101L, 102L, 103L, 104L, 105L,
                        106L, 107L, 108L, 109L, 110L, 111L, 112L, 113L, 114L, 115L,
                        116L, 117L, 118L, 119L, 120L, 121L, 122L, 123L, 124L, 125L,
                        126L, 127L, 128L, 129L, 130L, 131L, 132L, 133L, 134L, 135L,
                        136L, 137L, 138L, 139L, 140L, 141L, 142L, 143L),
     Identification = "LONNIE POOL FIELD WEAVERVILLE",
              Dates = c("1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014",
                        "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014",
                        "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014",
                        "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014",
                        "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014",
                        "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014",
                        "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014",
                        "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014",
                        "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014",
                        "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014",
                        "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014",
                        "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014",
                        "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014",
                        "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014",
                        "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014",
                        "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014", "1/01/2014",
                        "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014",
                        "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014",
                        "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014",
                        "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014",
                        "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014",
                        "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014",
                        "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014",
                        "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014",
                        "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014",
                        "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014",
                        "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014",
                        "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014",
                        "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014",
                        "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014",
                        "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014", "2/01/2014",
                        "2/01/2014", "2/01/2014", "2/01/2014"),
       TEMP_Celcius = c(13L, 10L, 8L, 7L, 5L, 4L, 3L, 3L, 2L, 2L, 2L, 2L, 0L, 0L,
                        0L, 0L, 0L, 0L, 0L, 0L, -1L, -1L, -2L, -1L, -2L, -2L,
                        -2L, -2L, -2L, -2L, -2L, -2L, -3L, -3L, -3L, -3L, -3L, -3L,
                        -3L, -3L, -4L, -4L, -3L, -4L, -4L, -4L, -4L, -4L, -4L, -3L,
                        -3L, -2L, 0L, 1L, 2L, 3L, 4L, 6L, 6L, 8L, 9L, 9L, 10L, 11L,
                        12L, 12L, 12L, 12L, 13L, 13L, 13L, 13L, 12L, 10L, 9L, 8L,
                        6L, 5L, 5L, 4L, 4L, 3L, 3L, 2L, 2L, 2L, 2L, 0L, 1L, 0L, 0L,
                        0L, 0L, 0L, 0L, 0L, 0L, 0L, 0L, -1L, -1L, -1L, -2L, -2L,
                        -1L, -2L, -2L, -2L, -2L, -2L, -2L, -2L, -2L, -3L, -3L, -3L,
                        -3L, -3L, -3L, -3L, -3L, -2L, -2L, 0L, 0L, 1L, 3L, 4L, 5L,
                        6L, 7L, 8L, 9L, 10L, 10L, 12L, 13L, 13L, 13L, 13L, 14L, 14L,
                        14L))
  </code></pre>
  
  <p>And I need to get others columns, like average temp, min, max, and average dew point max and min each per day. 
  I have many results per day because it is hourly. 
  I tried many ways but I am getting a wrong result.</p>
  
  <p>First I tried to get the mean with this code:</p>
  
  <p><code>tapply (Dados$TEMP_Celcius, Dados$Dates, mean)</code>
  But I get wrong result. For example for the date 01-01-2014 I get 27.8 and the right result would be 1.97.</p>
  
  <p>I tried either the codes:</p>
  
  <p><code>tapply(Dados$TEMP_Celcius, Dados$Dates, mean, na.rm = TRUE)</code></p>
  
  <p><code>aggregate(Dados$TEMP_Celcius, by=list(TMEDIA=Dados$Dates), mean)</code></p>
  
  <p>But I got the same results. I do not know what I am doing wrong, could you help me please?</p>
  
  <p>I already checked the class of the column Dates and it is "Date" and of teh variable temperature and it is "numeric".</p>