# To exclude or include the intercept in GLM model [duplicate]

When is is appropropriate to include or exclude the intercept from a regression model? SPSS provides this option in the GLM menu.

I am assessing group difference (gender) on tasks performed (X) and income earned (Y). In this case, if no task is performed, then no income is earned.

The SPSS guidelines states here:

"Include intercept in model. The intercept is usually included in the model. If you can assume that the data pass through the origin, you can exclude the intercept."

My stats knowledge is a bit rusty now but I remember my teacher saying that you should never exclude the intercept!

## marked as duplicate by kjetil b halvorsen, John, Michael Chernick, mdewey, gung♦ regression StackExchange.ready(function() { if (StackExchange.options.isMobile) return; $('.dupe-hammer-message-hover:not(.hover-bound)').each(function() { var$hover = $(this).addClass('hover-bound'),$msg = $hover.siblings('.dupe-hammer-message');$hover.hover( function() { $hover.showInfoMessage('', { messageElement:$msg.clone().show(), transient: false, position: { my: 'bottom left', at: 'top center', offsetTop: -7 }, dismissable: false, relativeToBody: true }); }, function() { StackExchange.helpers.removeMessages(); } ); }); }); Sep 18 '17 at 14:25

• The question to ask is not whether $(0,0)$ lies on the curve ("no task performed, then no income is earned") but whether it lies on the extrapolation to the origin of the rest of the data. If not, then $(0,0)$ does not fit the model, so forcing the curve to go through it would be a mistake. By analogy, consider a person surveying the top of a cliff bordering the ocean. When they map the elevations, it would be foolish (and dangerous) to force the ground surface to meet the ocean (and thereby erase the cliff): people could get killed that way! – whuber Feb 24 '12 at 16:24