Should I fire this person?

I have a business and I have two employees. One has been with me for 15 years and she is extremely valuable. The other is part-time and works three days a week. I administer the Kolbe test to anyone I hire and I have taken it. It’s only 35 questions and there are no right or wrong answers. It measures how we instinctively solve problems if left to our own means. We are closely associated with another organization which also uses Kolbe. The bottom line benefit of Kolbe is it helps put everyone in the right job, the one they are best at and will enjoy the most. We all, including me, share our scores with each other, so that we can better understand each others’ strengths.

After the part-time employee had been hired and had worked for several months she was asked to take the Kolbe test, and she refused. No good reason. She just didnt like it. I readily admit that I should have made it a condition of employment before I hired her. We had a lot going on and it slipped through the cracks. But since it has only a positive effect on the working environment, I never dreamed she would just refuse.

Question now is, should I fire her? Job performance is adequate, but not sparkling, by any means. And I would not fire her for refusing to take the test. But I would have no problem finding a reason.

Does she stay or go?