To say that data is theory laden is to say that what is observed (what data is acquired) is influenced by the scientific theory being assumed while making those observations.  There is a discussion of theory ladeness of data in the SEP article on observation, and in a web page on N. Hanson and his ideas on the issue.  It is also mentioned in the Wikipedia page on philosophy of science.

To me, it seems obvious that much scientific data is theory laden.  However, it turns out that the idea of theory ladeness of data is controversial.  When I look at the web page that I linked above, I can see why it is considered controversial.  Apparently, philosophers tend to look at theory ladeness of data as an example of cognitive bias that might cast doubt on the science.  I see that as a mistaken way of looking at it.  In this post, I plan to discuss why theory ladeness is to be expected, based on the way that science works.

In an earlier post on the nature of scientific law, I used Boyle’s law and Ohm’s law as illustrations.  I plan to use those examples again in this discussion of theory ladeness.

Let’s start with Boyle’s law, which asserts that PV is constant.  Here, P is the gas pressure, while V is its volume.  The underlying assumption is that the temperature is constant and that we are dealing with a fixed mass of gas (or a fixed collection of gas molecules).  As mentioned in the earlier post, Boyle’s law is an idealization.  It is true for an imagined ideal gas, but not for any actual gas.  It is a good approximation for actual gases, and is closest to being true for gases at low density.

The two quantities, pressure and volume, that are related in Boyle’s law are quantities that were known before Boyle’s law was formulated.  And, because of this, there is no reason to believe that the data is theory laden.  In a specific case, an investigator might find it easier to measure only pressure and to use Boyle’s law and other information to infer the volume.  In such a case, we could say that the volume data is theory laden.  But in that case, we would typically consider only the pressure readings to be data and those would not be theory laden.

Boyle’s law, and the other ideal gas laws, are particularly important for mathematically modeling the behavior of bodies of gas.  That the law is an idealization, so not precisely true, is not particularly important in such models.  That the gas laws are mathematically simple, and are good approximations to gas behavior, even if not exactly true, is what matters.

The situation with Ohm’s law is very different.  The laws says that V = IR, where V is the electrical voltage, I is the electrical current and R is the circuit resistance.  There are other electrical laws, so that data on V and I might sometimes be described as theory laden with respect to those other laws.  However, for the purposes of this discussion, we are concerned only on whether data is laden with the theory presented in Ohm’s law itself.  Since voltage and current existed and were known prior to the formulation of Ohm’s law, we might reasonably conclude that data on voltage and current is not theory laden with respect to this law.

The situation is different for the quantity R, or electrical resistance.  Ohm’s law is normally taken to be the definition of R.  And, because of that, data on resistance is unavoidably theory laden with respect to Ohm’s law as theory.

If we were just measuring V and I, and then using Ohm’s law to infer the value of R, then it would be proper to say that the measurements of V and I were the data, and that those were not theory laden.  In practice, however, it is often useful to just measure the resistance.  For that, we use an Ohmmeter or a multimeter that is switched to the configuration for reading resistance.  Measured that way, we are directly measuring R, rather than inferring it from measurements of V and I.  The relations in Ohm’s law are built into the measuring equipment.  When we are directly measuring resistance, the theory ladenness of the resistance data is unavoidable, and in fact is necessary in order for it to be possible to measure resistance.

Ohm’s law is important because it gives us the ability to measure new information that was previously not being collected at all.  It is like striking a mother lode of information.  And that’s what makes it important and useful in predicting and controlling the behavior of electrical circuits.