I have 10 pennies for every year. I have 10 different years. so total of 100 pennies and have their weights. I have to plot average weights for each year and I have to put 3σ error bars. How do I calculate the 3σ error bars? I don't get the 3σ part. I calculated the standard deviations stdev.p for weights of each year and when I put them on the graph, it looks weird. I don't even know if it's supposed to be stdev.p or something else.