Yoga Nidra is a type of yoga that helps ease the transition into a meditative state between sleeping and wakefulness. It is praised for its ability to induce sleep relaxation, reduce anxiety, and promote better sleep. And there's some evidence to support those claims. In a 2018 International Journal of Yoga study, 100 adult participants who did yoga Nidra and regular seated meditation for 45 minutes, twice a week for three months had lower levels of anxiety, stress, and depression compared to volunteers who didn't practice them. While both were beneficial, the researchers found that yoga Nidra was easier to perform and showed statistically significant results of increased levels of the "happy hormone" dopamine, which helps regulate mood, attention, and sleep.

Part (a): Was this study an experiment or observational study? Explain.

Part (b): Is the researcher able to make the conclusion that yoga Nidra increases dopamine and lowers anxiety for all adults? Explain.

Part (c): How could the researchers have changed their study to make it a completely randomized design? Identify the changes needed for the researchers to be able to draw conclusions about the entire population.