Describing phenomena in a system definition is a basic step for understanding, assessing, and modelling socioeconomic metabolism. Depending on the research question at hand, we model phenomena in the industrial system as stocks within and flows between processes in the system. The stock-flow distinction is crucial to understanding how the different methods model reality and for correctly defining performance indicators for the systems we analyse. Let’s see what GPT  has to say about this topic and whether we can use its output to learn something about the topic and discover common misconceptions!
Below, I document snippets from a dialogue with the March 23 version of ChatGPT on the topic of stocks and flows. The GPT replies posted below are copied 1:1, but I shortened some of them as some replies are rather long without much additional content. I added my evaluation of the replies as [comments].
Q: In material flow analysis, what is a stock and what is a flow? What phenomena do these concepts describe and how are they measured?
A: In material flow analysis (MFA), stocks and flows are fundamental concepts used to describe the movement and transformation of materials within a system.
A stock in MFA refers to the amount of a material that is present within a system at a given point in time [Exactly! Plus that a stock is always placed within a process in the system].
It represents the accumulated quantity of a material that has been used or accumulated over time and is still present in the system. For example, the total amount of steel used in a building construction up to a specific point in time would be considered a stock. [not fully correct: the stock is the material that is (still) present at a given point in time, whereas the accumulated use over time (integral over an inflow of material up to a given point in time) might (partly) have left the system already and that part is not part of the stock]
A flow in MFA, on the other hand, refers to the movement or transfer of materials from one part of the system to another during a specific time period. Flows represent the inputs and outputs of materials into or out of the system. For example, the amount of steel used in the construction of new buildings during a specific year would be considered a flow. [Exactly! Flows represent the movement of materials between processes within the system or to and from the system’s environment from or to a process in the system]
Stocks and flows are typically measured in physical units, such as mass or volume, and can be quantified using various methods, including direct measurements, estimations, or modeling approaches. Common units for measuring stocks and flows in MFA include metric tons, kilograms, cubic meters, liters, etc. [Correct for stocks. For flows, one has to distinguish between flow value for discrete time intervals (same unit as stocks) and continuously measure flow rates per time (unit of stocks per time)]
It’s important to note that the distinction between stocks and flows can depend on the level of detail and scope of the MFA analysis. In some cases, certain materials or components may be treated as stocks within a process or system, particularly if the focus of the analysis is on the use, lifespan, or disposal of a specific product or material. The specific definitions and measurements of stocks and flows in MFA may vary depending on the specific application and context of the analysis. [true but not very specific: an example should be provided.]
[Overall: A compact definition is: A stock is a quantity of material measured inside a process at a given point in time, and a flow is a quantity of material moving from one process to another during a given time period.]
Q: Above, you write that it’s important to note that the distinction between stocks and flows can depend on the level of detail and scope of the MFA analysis. Provide an example where a phenomenon or event in reality is modelled as stock and one where it is modelled as a flow!
A: Certainly! Here are examples of phenomena or events in reality that can be modeled as either stocks or flows in the context of Material Flow Analysis (MFA):
Example of a phenomenon modeled as a stock: Carbon stored in a forest. In this case, the total amount of carbon stored in the forest biomass (e.g., trees, shrubs, etc.) at a specific point in time would be considered a stock. It represents the accumulated amount of carbon that has been sequestered or stored in the forest over time. [Correct description but again, the confusion between accumulated sequestration (integral over all sequestration, no matter whether it is still in the system or decomposed already) and the stock (accumulated sequestration remaining in the system) persists.]
Example of a phenomenon modeled as a flow: Carbon emissions from fossil fuel combustion. In this case, the amount of carbon dioxide (CO2) released into the atmosphere from burning fossil fuels during a specific time period (e.g., annually) would be considered a flow. It represents the rate of change or movement of carbon emissions over time. [Correct example. The last sentence is not correct: The rate of change of carbon emissions over time is the derivative of a flow (carbon emissions), and the concept ‘movement of carbon emissions over time’ does simply not make sense)]
Q: Consider the buildings (residential and non-residential) that we live and work in. Is that a stock or a flow?
A: Buildings, whether residential or non-residential, are considered a stock because they represent a fixed amount of physical infrastructure that exists at a particular point in time. Once a building is constructed, it remains as part of the stock until it is demolished or otherwise removed from use. [Exactly!]
However, it’s worth noting that there are some aspects of building construction and maintenance that could be considered flows. For example, the maintenance and repair of a building over time could be seen as a flow, as it represents ongoing upkeep and improvements to the building. Additionally, the construction of new buildings could be considered a flow, as it represents the ongoing creation of new infrastructure over time. [Exactly, we model new construction of buildings as flow (inflow into the use phase) and building repair and maintenance as additional inflows of materials into the use phase]
Q: In life cycle assessment (LCA), all capital is modelled as flows. How can that be, or how exactly does this work?
A: In life cycle assessment (LCA), all capital is modeled as flows rather than stocks. This is because LCA is a method used to assess the environmental impact of products or processes throughout their entire life cycle, which includes not only the production and use phase, but also the disposal and recycling phase. In order to model the flow of resources throughout the life cycle, all capital is treated as a flow, even though in reality it can be seen as a stock.
The reason for modeling capital as a flow in LCA is that it allows for a more accurate representation of the resource use and environmental impact of a product or process over time. By modeling capital as a flow, it is possible to account for the ongoing use and depletion of resources over the entire life cycle of the product or process, rather than just at a single point in time. This can help to identify areas where resource use can be reduced or where recycling or reuse of materials can be improved to minimize environmental impact.
In summary, all capital is modeled as flows in life cycle assessment (LCA) in order to account for the ongoing use and depletion of resources over the entire life cycle of a product or process. This allows for a more accurate representation of the environmental impact of the product or process and can help identify areas where resource use can be reduced or recycling can be improved.
[Correct answer but the explanation is unsatisfactory. LCA models the production of single products, and it aims at including all resource input needed for single products. It therefore considers only the consumption of fixed capital stocks that corresponds to the production of a single product to be part of the product system. For example, to drive one km with a car, you need an entire car but it is available for further use after the ride. The appropriate consumption of a car for a 1 km ride would be 1/150000 units, if the assumed total service generated is 150000 km throughout the car’s lifetime. ]
Q: In psychology, the term ‘stock flow failure’  is used to refer to situations where it is hard for humans to distinguish stocks from flows. Give an example based on the discussion on MFA and LCA above!
A: Sure! In the context of Material Flow Analysis (MFA) or Life Cycle Assessment (LCA), a “stock flow failure” might refer to a situation where it is challenging for humans to accurately distinguish between stocks and flows of materials or resources. Here’s an example:
Let’s consider the case of a building’s carbon footprint assessment using LCA. In the use phase of the building, the consumption of electricity from the grid can be considered a flow, as it represents the rate of electricity consumed over time. On the other hand, the embodied carbon in the building materials, such as concrete and steel, can be considered as a stock, as it represents the total amount of carbon emissions associated with the production and transportation of those materials. [The latter example does not work. Embodied carbon emissions are a flow from the product system to the environment, which is inherently clear from the term ‘emissions’, which is an activity (emission of GHG from industrial processes) that is modelled as a flow.]
[The term ‘stock flow failure’ was coined by Cronin et al. (2009) , who identify an often poor understanding of accumulation of flows in stocks, revealed by assuming that the behavior of a stock matches the pattern of its flows.]
Q: Apply the following definition in all further responses in this chat: “: A stock is a quantity of material measured inside a process at a given point in time, and a flow is a quantity of material moving from one process to another during a given time period.” Using greenhouse gas (GHG), give an example for a GHG stock and a GHG flow!
A: Certainly! Here’s an example using greenhouse gases (GHG):
GHG Stock: The total amount of carbon dioxide (CO2) emitted and accumulated in the atmosphere over a period of time can be considered as a GHG stock. It represents the quantity of CO2 that is present in the atmosphere at a specific point in time, and it can be measured using atmospheric monitoring stations or estimated through modeling. [Again, only partly correct, as ‘emission to’ is not the same as ‘accumulation in’, because GHG are decomposed or leave the atmosphere. Correct example of atmospheric CO2 stock.]
GHG Flow: The rate at which carbon dioxide (CO2) is emitted from a power plant during its operation can be considered as a GHG flow. It represents the quantity of CO2 that is released into the atmosphere from the power plant during a given time period, such as per hour, per day, or per year. It can be measured using emission monitoring systems or estimated through emissions factors and operational data. [Correct!]
Summary: Overall, GPT delivers a large volume of highly specific and relevant text in a very short time. The content delivered is largely correct, which is astonishing given the subtlety of the stock-flow distinction and how poorly it is documented in the literature. Some crucial distinctions (cumulative flows vs. stocks, process definition, or the correct explanation of stock-flow consideration by the different methods) are not well described, which may stem from a lack of correct description of these issues in the training data. Hopefully, this blog entry will help practitioners, scholars, and chat robots do it better in the future! Expert knowledge is still needed to evaluate the correctness and usefulness of the text generated.
The above questions and answers could be taken from an exam protocol. My judgement here would be that the candidate has a good understanding of stock and flow modelling but that some crucial aspects (especially the differences between cumulative flows and a stock and the application in different methods) still have to mature further.
 https://chat.openai.com March 23 version of ChatGPT, accessed March/April 2023.
 Why don’t well-educated adults understand accumulation? A challenge to researchers, educators, and citizens, by MA Cronin et al. Organizational Behavior and Human Decision Processes, 108(1), 2009, pp 116-13, https://doi.org/10.1016/j.obhdp.2008.03.003