Posted by Debbie Weissmann
Our March Chapter Meeting was a PD led by Colleen Lewis, who had been one of our IE chapter leaders, but was enticed away by U of Illinois, Urbana Champaign. Colleen shared teaching methods to help students see the underlying structures of CS problems. In this PD she used Scratch to explore common patterns of iteration. Here is a link to the PD  Here is a link to her slides
Colleen started by describing the issue to be addressed: Many students are not able to see the underlying structure of CS problems. She gave the example of loops. Colleen demonstrated on Scratch how we can explicitly set a variable, use it, change it, and repeat the process over and over. She also highlighted implicit changing variables with pen sizes and musical notes. 
She applied the iteration examples to make patterns such a star, a snowflake, and petal. She demonstrated how she presents the lesson for her students in which they can see the underlying structure of the loop become visible. 
Taking it a step further, Colleen Shared the relationship between loops and Big O. (Big O is a theoretical model to classify algorithms according to run time or how much memory they take up.)  In watching the patterns emerge in Scratch one could visualize the impact of Big O on programs that included nested loops. We all then watched a couple of fun YouTube videos that provided visualizations of Big O using common sort methods, and of President Obama demonstrating his expertise in CS
Thank you Colleen Lewis for sharing your teaching methods with us!