After World War II, the US were panicking by the idea of Europe falling to communism. One focal point was the labor union. With the experience of the labor unions in their own country, they were convinced that the European labor unions falling into communist hands was a real life prospect.
Luckily they understood little of the polder economics of the Netherlands. There, captains of industry were dining at the same table as the union leaders, looking for a solution acceptable to both. Luckily - for my father. Having a two year education as a precision mechanic at a lower vocational technical school, riding his bike through post-war Rotterdam as a typewriter repair man, he was selected to follow a higher education at a labor union school. It sounds strange, but it's all part of the typical Dutch compartmentalization of the country: Protestants, Catholics, socialists, they all had their own infrastructure. Their own schools, their own unions, their own political parties, their own banks, insurance companies, you name it.
But the US knew nothing about this consent culture. It meant they put their money into the education of union leaders, preventing them to fall for the red danger, becoming communists.
So my father went to study at the Catholic A.C. de Bruijn-institute, under a dean (and priest) called Olierook (Oilsmoke). There he was educated and became a union leader, keeping faithful to his belief of consensus and decency all his life. He died in 1992, 64 years old, of a brain tumor.
His study was economic in nature, with lessons about productivity, optimization of production etc. During his study he was selected to follow an extra one year course to become a productivity- and efficiency-consultant. And this course came to be because of allocated Marshall money. What you see in the picture above is one of the results: a (German!) slide ruler. It now belongs to me.
It's an Aristo nr.99, from 1953 (G531). The function of the slide ruler is based on the principle of logarithms: $$^a log(b)+ ^a log(d)=^a log(b\cdot d)$$ and all its variations.
To this day I'm amazed about the accuracy of the slide ruler. This one is a big one, making the scale bigger and better to read. And of course, as it is a logarithmic scale, answers are harder to read at the end of the scale, because of the logarithmic compression at the higher end of the scale.
With this slide rule I get an average accuracy of between 0.15 to 0.25% for every end of the scale. In the above example I calculated $$153\cdot 252\approx 41600$$ That's even more accurate than 0.15%, as $$153\cdot 252= 41616$$ and that's an accuracy of less than 0.04%.
In today's world that accuracy is laughable. And in pharmaceutics it can be deadly. Besides, you have to know your decimals. If you don't anticipate the answer to be in the neighborhood of $200\cdot 200 =40,000$ you might end up with "maybe the answer is 416 or so."
And that might be laughable as well, but it sure is reality nowadays. Ladies and gentlemen: start - your - calculators!