This approach can be particularly powerful if you have one figure displayed as the audience is settling rather than an opening title and name slide. This ensures that the audience has already built up an interest in what you are going to say by trying to decide amongst themselves what the number / statistic relates to. It’s a bit like a crime program leaving you until after the break or until another night to conclude the ‘mystery’. The only rule of thumb is that your statistic must directly relate to your audience and your main presentation aim / message.