Let's say that there is a study that focuses with the effect of storage times on the calcium content of barley. Samples of barley were stored for different times (either $0,1,2,$ or $4$ months) and the calcium content was measured (grams of Ca per kg of barley). We suppose that this study is balanced and a total of $35$ samples were taken.
Initial Ideas
Our subjects (or units) would our samples of barley, our treatement would be the different storage times ($0,1,2,$ or $4$ months) and lastly, our response (or conclusion) would be the amount of calcium content that was collected from the barley.
Based on the description above, one of the questions had to do with whether this study was a design experiment or an observational study and I chose the former because the researchers were in control of assigning the treatments to our subjects. How does this sound?
Regards, John
In an experiment investigators apply treatments to experimental units (people, animals, plots of land, etc.) and then proceed to observe the effect of the treatments on the experimental units.
In an observational study investigators observe subjects and measure variables of interest without assigning treatments to the subjects. The treatment that each subject receives is determined beyond the control of the investigator.
As such, yours is clearly an experiment, since you are exploring effect of treatments.