Dr. Smyth is testing to see whether preschool children learn new words faster with pictures or objects. He wants to test each child under both conditions. He has all the children participate in the picture condition and then in the object condition and finds the object condition to produce better results. What flaw in his design could provide an alternate explanation for his findings

He makes all the children participate in learning through picture & object tools both. And, concludes that object tools condition gives better results.

However, his design & conclusion is flawed. As, he overlapped the same group of children to test through both the learning methods (pictures & objects).

When children are indulged in both methods, their learning is likely to be cumulative effect of both the methods. Its very difficult to segregate the learning impact owing to individual method & compare them.
Creating different student groups, for learning through the two methods - is a better option. Two distinct groups taught through the 2 different methods, & their learning analysis would give better study result about the methods' comparative efficiency.