How Do I Set Focus on Dropdown Lists?

I'm trying to set focus to various fields when users neglect to enter required information. It works great on textbox fields, but not on dropdown lists.

Here's the code I'm using:

if (showingDate == null || showingDate == '')  {   alert('Please enter a Showing Date (Scheduled), or change the Reason for the Call to Property Question.');   document.getElementById('custevent_showing_date_scheduled').focus();  }    else if (showingTimeStart == null || showingTimeStart == '')  {   alert('Please enter a Showing Start Time (Scheduled), or change the Reason for the Call to Property Question.');   document.getElementById('custevent_showing_time_scheduled').focus();  }    else if (showingTimeEnd == null || showingTimeEnd == '')  {    alert('Please enter a Showing End Time (Scheduled), or change the Reason for the Call to Property Question.');   document.getElementById('custevent_showing_end_time').focus();  }    return false;