For successful seed germination, sowing 3 seeds per cell is usually a good rule of thumb. This generally guarantees at least one of the seeds will germinate. When planting lettuce seeds or other small seed crops like flowers, the seeds are so small that often times you are dropping in far more seeds into each planting hole than planned. This often results in many sprouts emerging from the soil at once. When these seedlings are crowded, they will not grow to their mature size, which results in a smaller harvest. An effective way to increase your yield by giving your crops sufficient space to grow is by thinning your seedlings.
The best time to thin your lettuce seedlings is when plants have reached about 2-3 inches in height. At this stage of growth, the root systems are usually strong enough to be pulled up without disturbing the growth of roots being kept in the ground. You can also remove them with a knife, but I like to pull the roots up as I believe it gives that much more space for growth of the lettuce you are keeping in the ground. Remember to keep your largest and healthiest lettuce seedling in the ground to grow to maturity. Always remove the smaller seedlings around it.
You can yield a surprising amount of food from thinning your greens. From a 2'x4' space I was able to harvest almost 8 oz of greens. It made a large salad for dinner for my family. This will be the first of many! You can also harvest baby carrots and beet tops with this same thinning method.