If you are showing statistical data, you are probably using an equal area projection. For most maps made with these projections, it is inappropriate to use a north arrow, since north is only "up" along the central meridian of the projection. This becomes even more evident for larger areas, so unless the extent of your study area is relatively small (say about the size of a U.S. county), you will probably want to remove the north arrow. You could add the graticule (latitude-longitude grid) if you think your readers really need an orientation indicator. However, if your map is like most, north is at the top for the center of the map. Most people already understand this and most thematic maps are not used to measure direction, so you can leave an orientation indicator off your map entirely. Hope this helps!