Counting unique values in Excel : <p>I have a list that looks like this:</p>

<pre><code>Location   ID
______________
Boston     12
Boston     12
Boston     12
Boston     57
Boston     99
Chicago    12
Chicago    13
...
</code></pre>

<p>For each Location I want to count up the number of unique IDs. Boston would have 3 and Chicago would have 2 for example.</p>

<p>I can use Advanced filter by unique records but I want to know how to do this using an Excel formula.</p>
