How would the employment distribution by industry (primary, secondary, tertiary, quaternary) be different in 1920 from those in 2015 and what are the reasons for this change?
If this question is asking about the United States or any other developed nation, the answer is that the employment distribution in 1920 would have been much more heavily concentrated in the primary and secondary sectors of the economy than it is today. Today, workers in the most developed countries of the world are generally involved in the tertiary or quaternary sectors. There are those who work in the primary and secondary sectors, but they are much fewer than they were in 1920.
In the past, the world was much less technologically advanced than it is today. This meant that much more human labor was required to extract raw materials from nature and to turn those raw materials into goods that people could consume. Therefore, there were many more people working in the primary and secondary sectors in 1920. As you can see in this article, somewhere between 25 and 30 percent of American workers in the 1920s worked in the agricultural sector. Since there are other industries (mining for example) that are part of the primary sector, we know that at least one-third of all Americans worked in the primary sector in 1920. Similarly, manufacturing was much less technologically advanced than it is today. There was much less automation in factories and factories therefore needed many more workers than they do today. This would have meant that more people would be employed in the secondary sector in 1920 than today.
By contrast, the tertiary and quaternary sectors employ a higher percentage of workers today than they did in the past. This is partly due to the growth of technology and partly to the increased educational levels of workers in rich countries. It is also partly due to increased globalization. Today, technology has made it so that fewer people are needed to work in the primary and secondary sectors in rich countries. Many of these jobs have been taken by machines. In addition, many such jobs have gone overseas to places where labor is cheaper. At the same time, workers in rich countries like the US are much more educated today than they were in 1920. This means that they are more able to work in more advanced jobs such as those found in the quaternary sector. As you can see from the link below, about 80% of all American workers today work in services, which make up the tertiary and quaternary sectors.
In rich countries, technological change has destroyed jobs in the primary and secondary sectors and has freed up workers to become more educated and to get jobs in the tertiary and quaternary sectors. Therefore, the employment distribution today is more heavily weighted toward the two latter sectors than it was in 1920.