Interesting there is scientific evidence that an animal will work harder for an intermittent food reward. They tested how well animals complied with commands when rewarded
1. Every single time
2. At set regular intervals, i.e. every 2nd/3rd/4th time the animal displayed a behaviour
3. At random

The animals who were rewarded at random, displayed the desired behaviour more reliably/frequently. Apparently if you reward them every single time then the animal can become complacent because they know they will just get something the next time, so it doesn't matter so much if they miss this one. However, when rewards are random, the animal is less likely to want to miss an opportunity, in case this time is the jackpot!

Of course you need to reward all of the time when training until the behaviour is reliable. Claire has just said that she 'rewards' every time, she hasn't said food reward, and the above study was only based on food rewards only. I don't know what Claire does, but she might do a mix of toys, food and praise? It is a good idea to at least work towards a varied tariff of reward which rank different for the dog (say dry kibble, praise, ball, squeaky ball, cheese, sausage) working up to the best reward. That will motivate your dog as they never know what they are going to get!

I just thought I'd share, as I thought it was intersting that occassionally skipping the reward can make your dog want to work even harder

Yes a variable schedule of reinforcement is useful in all training scenarios.