Like it, but I don't the round() and divide() method since they introduce imprecision when it's not required.

I'd do something like Wikipedia says we should do to calculate sample median:

Easy explanation of the sample median

In individual series (if number of observation is very low) first one must arrange all the observations in ascending order. Then count(n) total number of observation in given data.

If n is odd then Median (M) = value of ((n + 1)/2)th item term.

If n is even then Median (M) = value of [((n)/2)th item term + ((n)/2 + 1)th item term ]/2

For an odd number of values

As an example, we will calculate the sample median for the following set of observations: 1, 5, 2, 8, 7.

Start by sorting the values: 1, 2, 5, 7, 8.

In this case, the median is 5 since it is the middle observation in the ordered list.

The median is the ((n + 1)/2)th item, where n is the number of values. For example, for the list {1, 2, 5, 7, 8}, we have n = 5, so the median is the ((5 + 1)/2)th item.

    median = (6/2)th item
    median = 3rd item
    median = 5

For an even number of values

As an example, we will calculate the sample median for the following set of observations: 1, 6, 2, 8, 7, 2.

Start by sorting the values: 1, 2, 2, 6, 7, 8.

In this case, the arithmetic mean of the two middlemost terms is (2 + 6)/2 = 4. Therefore, the median is 4 since it is the arithmetic mean of the middle observations in the ordered list.

We also use this formula MEDIAN = {(n + 1 )/2}th item . n = number of values

As above example 1, 2, 2, 6, 7, 8 n = 6 Median = {(6 + 1)/2}th item = 3.5th item. In this case, the median is average of the 3rd number and the next one (the fourth number). The median is (2 + 6)/2 which is 4.

Code:

List<Integer> testvalues = new List<Integer>();

// Insert some values
testvalues.add(1);
testvalues.add(6);
testvalues.add(2);
testvalues.add(8);
testvalues.add(7);
testvalues.add(2);

Integer sizeOfList = testvalues.size();
system.debug('size of list is '+ sizeOfList);
Integer index = sizeOfList - 1;
system.debug('the index is '+index);
Decimal median = 0.0;

// sort the list first
testvalues.sort();

//Calculate median
if (Math.mod(sizeOfList, 2) == 0) {
   median = (testValues[(index-1)/2] + testValues[(index/2)+1])/2;
}else{
   median = testvalues[(index+1)/2];
}

system.debug('the median is: '+median);