I originally wrote this post in 2016, before I started college. Overall, I’m still pretty happy with it! I think it illustrates the important principle that the quality of an analysis is only as good as the worse of your methods and data (otherwise known as GIGO). I’m glad I didn’t succumb to the temptation to run some fancy regressions on pretty terrible data.
I’m also quite surprised by the footnote that mentions being unable to access a paywalled paper. Alexandra Elbakyan’s bio mentions that Sci-Hub became popular worldwide in 2016. Wow, there are some ways the world has progressed in the last four years! (I tried finding that paper, and it took me less than ten seconds to go from DOI to pdf with Sci-Hub)
On the other hand: ‘Why I’m confident: This is FAO data, the most official source there is.’ Really, Jules? Official = trustworthy?
“How much kale was consumed in Africa in the past year?”, was another SPARC 2016 application question. Buoyed by my ability to survive poisoned cookies, I decided to take a crack at it.
Abstract
I used three different methods to estimate that the amount of brassica oleracea var. Acephala eaten by human beings in Africa in the past year was one to two megatonnes (10^9 kilograms). I am fairly sure that the amount eaten was between zero and eight megatonnes (MT).
Introduction
Getting data on kale consumption in Africa was like trying to eat cabbage soup with one chopstick. Like the soup, the numbers I came up with must be seasoned with large pinches of salt.
Anyway, let’s get started! I’m going to break the question into its components and explain how I resolved each one.
“How much / kale / was consumed / in Africa / in the past year?”
- How much
- I decided to answer in megatonnes (10^9 kilograms) because SI units are cool, and I don’t have enough significant digits for a smaller one.
- kale
- This is the tricky one. Acording to Wikipedia and Plant Resources of Tropical Africa 2: Vegetables [1] , Kale is defined as brassica oleracea var. acephala, which is to say, it is a cultivar of cabbage that doesn’t form a head, but remains as flattish leaves.
- Ethiopian kale, also called African kale, is brassica carinata, a different species, so I didn’t count it.
- Rape kale, brassica napus, is also a different species. Not counted.
- Sukuma wiki, on the other hand, a staple vegetable in eastern Africa, is variously described as ‘kale’, ‘colewort’, ‘very similar to kale’ and ‘not to be confused with kale’. I chose to count it because it is undoubtedly an non-headed variety of brassica oleracea. Also, Merriam Webster defines colewort as kale. (Wikipedia just redirects colewort to brassica oleracea.)
- Was consumed
- I interpreted consumed to mean eaten by humans. See method sections for issues this caused.
- In Africa
- Fortunately, Africa is an island, so no border issues. I love the Suez canal.
- In the past year
- I decided to estimate for calendar year 2015 instead, since one year is exactly enough time to even out any seasonal variation, and all my data is sufficiently ancient that it makes very little difference.
Methods
I used three different methods to calculate kale consumption. This section will outline each method and the issues that cropped up with each.
Method 1: Food and Agriculture Organisation Production Data
- 1. Get FAO Production data.[2]
- Unfortunately, FAO reports ‘Cabbages and other brassicas’ together.
- Also, the last year I could get data for was 2012. After that FAO switched to a new site, faostat3.fao.org, which I couldn’t get anything out of. Even Wikipedia uses 2012 data for its featured article on Cabbage, which made me feel a little better.
- 2. Add up tonnages from primarily kale-producing countries.
- Figuring out which these were was very hard. In the end I decided to pull them out of my hat.
- Vegetables says
-
[Leaf cabbage] types with tall plants grown for repeated leaf pickings are popular everywhere in East and Southern Africa but less common in central Africa and rare in West Africa.
-
- I assumed Kenya, Tanzania, Rwanda, Malawi, and the Dem.Rep. of Congo were the countries where brassica meant kale. I also added 0.3x the South Africa figure since kale is commonly grown there too, although the major crop is cabbage [3]. Other countries in East and South Africa were blanks in the dataset.
- I ignored the rest of Africa, hoping the errors would partially cancel, Fermi style.
- 3. Extrapolate 2015 figure from 2010, 2011, and 2012 data.
- The figures did not show any linear trend, or any trend I could eyeball, so I just went with a close round number.
- 4. Adjust for:
- Kale exported from Africa (insignificant compared to other errors)
- Kale eaten by livestock (insignificant)
- Kale used as biofuel (this is mostly Ethiopian kale, so never mind)
- Spoilage losses. According to Lenne & Ward (2010) [4]
-
Post-harvest losses in both countries, [Kenya and Tanzania] can be as high as 50% depending on the vegetable, weather conditions and distance from markets (Global Horticultural Assessment, 2005). More than 90% of national vegetable production is available for domestic consumption as fresh produce.
- I shall assume that, on average, 75% of kale production is actually consumed.
-
- Kale not reported in data. If FAO data doesn’t include subsistence farming kale which never makes it to market, that throws my hopes of using it as an upper bound right out the window. I couldn’t find a satisfactory answer to this question, but Lowder et. al. (2014) [5] says
-
The FAO’s theoretical definition of an agricultural holding is “an economic unit of agricultural production under single management comprising all livestock kept and all land used wholly or partly for agricultural production purposes, without regard to title, legal form, or size. Single management may be exercised by an individual or household, jointly by two or more individuals or households, by a clan or tribe, or by a juridical person such as a corporation, cooperative or government agency’’ (FAO, 2005). FAO encourages countries to use an operational definition based on this theoretical definition when carrying out their agricultural census.
- I hope this means they do include kale people eat from their own fields.
-
- Let’s take a final correction factor from production to consumption of 75%.
- 5. Check for mistakes. Is my answer in the right units? Does my answer make sense?
Method 2: Cultivated Area x Yield
- 1. Find Area under Kale cultivation.
- The only non-FAO pan-Africa source I could find was Vegetables [1] which says
-
The total area planted [ in Africa] may amount to over 100,000 hectares but no precise data are known. A conservative estimation for the area planted with leaf cabbage [in Zimbabwe] is 2500 ha for commercial crops and 2500 ha for subsistence crops, as most rural households grow leaf cabbage for family use.
-
- FAO brassica data for Zimbabwe is 35 hectares, which differs by two whole orders of magnitude. Not a good sign.
- The only non-FAO pan-Africa source I could find was Vegetables [1] which says
- 2. Find Yield. I can’t use FAO yields, because those are calculated, not independently measured.
- Vegetables [1] says
-
Average yield is 20t/ha for a once-over harvest and estimated yield is 50t/ha from 10 pickings in 6 months
-
- South Africa has the highest cabbage yield in the world at 64 tonnes/hectare/year[6]
- Let’s take 30 tonnes / hectare/year as our estimate. We take 20t/ha and 100t/ha as upper and lower bounds.
- Vegetables [1] says
- 3. Multiply yield per hectare per year by area under kale cultivation.
- Kale is not native to Africa, so I shall assume there isn’t much uncultivated wild kale for people to eat.
- 4. Adjust from production to consumption, the same way as method 1. Multiply by 75 percent.
- 5. Check for mistakes.
Method 3 Calorie Counting
- 1. Find data on vegetable consumption per capita in Africa. Actually the other way around, I found this data first, then thought of this method.
- According to Grubben[7] and Shackleton[8], average vegetable consumption in sub-Saharan Africa is 100-150 gm per day. This is significantly lower than WHO recommendations of 400 grams of fruit and veggies a day, which seems about right.
- Food Production and Consumption Trends in Sub-Saharan Africa [9] gives vegetable consumption in kilocalories per person per day as roughly 35 (when extrapolated to 2015).
- Kale provides 50 kcal per 100 gram serving, so 500 kcal/kg.
- 2. Assume a population which gets all its vegetable calories from kale.
- Mariga et al. [10] say that
-
Foeken and Owuor (2008) reported that nearly every household of the low-income dwellers of Nakuru, Kenya, survived on B. oleracea var. Acephala.
-
- I shall take the sum of the current populations of Kenya and Tanzania. Naturally, not every person in these countries eats no other vegetables, but hopefully they will cancel out the many kale eaters outside Kenya and Tanzania.
- Mariga et al. [10] say that
- 3. Multiply everything together to get a final answer in megatonnes of kale per year
- 4. Check for mistakes.
Results
Method 1: Food and Agriculture Organisation Production Data
| Country | Crop | Production in tonnes | Production in tonnes | Production in tonnes | |||
| Democratic Republic of the Congo | Cabbages and other brassicas | 26000 | F | 26867 | Im | 27500 | F |
| Kenya | Cabbages and other brassicas | 784876 | 599625 | 684000 | |||
| Malawi | Cabbages and other brassicas | 87357 | Im | 77012 | Im | 78500 | F |
| Rwanda | Cabbages and other brassicas | 133120 | Im | 117355 | Im | 120000 | F |
| Zimbabwe | Cabbages and other brassicas | 460 | Im | 406 | Im | 450 | F |
| South Africa adjusted | 39936 | 35206.5 | 36000 | ||||
| United Republic of Tanzania | Cabbages and other brassicas | 45000 | F | 39671 | Im | 42000 | F |
| Total | 2010 | 1116749 | 2011 | 896142.5 | 2012 | 988450 |
F means FAO estimate. Im = FAO data based on imputation methodology
Since there doesn’t appear to be much of a trend, I may as well round it off to 1 megatonne.
Adding together all the twenty-five countries in the FAO dataset, not just these seven, gives us an upper bound of 3 megatonnes.
Adjusting for 25% losses gives .75 MT and 2.25 MT.
Method 2: Cultivated Area x Yield
| Area(hectares) | Yield (t/ha) | Product in tonnes | Adjusted | ||
| 100000 | 30 | 3000000 | 0.75 | 2.25 MT | Estimate |
| 20 | 2000000 | 0.75 | 1.5 MT | Lower bound | |
| 100 | 10000000 | 0.75 | 7.5 MT | Upper bound |
Method 3: Calorie Counting
| Factors | |
| 1 kg kale/500 kcals | |
| * | 35 kcal/person/day |
| * | 365 days/year |
| * | 93,063,000 people (Kenya+Tanzania) |
| * | 1 megatonne/ 1000,000,000 kgs |
| equals | 2.37 MT of kale consumed a year |
Conclusion
Whew! That was fun, but quite tiring. Now to discuss my confidence in each method. In short:
Method 1: Unreliable
Method 2: Highly unreliable
Method 3: Okay, I admit it. I just did this one for the fun of a xkcd What-If? style dimensional analysis.
Method 1
Why I’m confident: This is FAO data, the most official source there is.
Why I’m unconfident: Counting all brassicas together is a major source of error. My kale estimate could be off by about 50%. If the FAO doesn’t count subsistence farming properly, the true production could easily be twice or thrice my estimate. I was rather disturbed by the fact that Nigeria and Uganda were nowhere to be seen in the FAO dataset. Also the figure for Zimbabwe is suspiciously low, barely 450 tonnes.
Method 2
Why I’m confident: The book is talking about kale specifically.
Why I’m unconfident: It’s so different from the FAO data!
Method 3
Why I’m confident: Here I directly find consumption, not production, which removes many sources of error.
Why I’m unconfident: This is a Fermi estimate, with all the hand-waviness that implies. I assumed pretty much everything for the population. I could easily have taken just Kenya, or all the kale producing countries, or anything. I really should have taken Uganda, but I didn’t because of its absence from the FAO data.
My three estimates were .0.75 MT, 2.25 MT and 2.4 MT. I estimate their relative validity as 55%, 40% and 5%. Averaging using the above weights gives us:
| Estimate | Weightage | MT | |
| Method 1 | 0.75 | 0.55 | 0.4125 |
| Method 2 | 2.25 | 0.4 | 0.9 |
| Method 3 | 2.37 | 0.05 | 0.1185 |
| Total | 1 | 1.431 |
Phew! After all that, we get 1.4 MT.
I don’t think we can use two significant figures, so I’ll call that a final answer of 1-2 megatonnes of kale consumed in Africa in the past year.
I estimate a 50% chance that the true value lies within that range.
For an upper bound, we can use the optimistic 7.5 megatonnes we got using the second method.
I estimate a 95% chance that the amount of kale consumed was less than 8 megatonnes.
Bibliography
[1] Mvere and van der Werff, 2004. Brassica Oleracea L. In: Grubben & Denton (Editors) Plant Resources of Tropical Africa 2: Vegetables. PROTA Foundation.
[2] FAO Crop Production Dataset at http://faostat.fao.org/site/567/DesktopDefault.aspx?PageID=567#ancor. Search Parameters: Africa-Cabbage and other brassicas-2010, 2011, 2012-Production, Area under Cultivation.
[3] Mandiriza-Mukwirimba Quenton Kritzinger and Theresa Aveling. A survey of brassica vegetable smallholder farmers in the Gauteng and Limpopo provinces of South Africa Journal of Agriculture and Rural Development in the Tropics and Subtropics Vol. 117 No. 1 (2016) 35–44.
[4] Improving the efficiency of domestic vegetable marketing systems in East Africa: Constraints and opportunities. J.M. Lenné and A.F. Ward Outlook on AGRICULTURE Vol 39, No 1, 2010, pp 31–40.
[5] Lowder, S.K., Skoet, J. and Singh, S. 2014. What do we really know about the number and distribution of farms and family farms worldwide? Background paper for The State of Food and Agriculture 2014. ESA Working Paper No. 14-02. Rome, FAO.
[6] Vegetables I: Asteraceae, Brassicaceae, Chenopodicaceae, and Cucurbitaceae edited by Jaime Prohens-Tomás, Fernando Nuez.
[7] Grubben, G., W. Klaver, and Non-Womdim R. Vegetables to combat the hidden hunger in Africa (2014) In Chronica Horticulturae Volume 54 Issue 1 Pagination 24 – 32.
[8] African Indigenous Vegetables in Urban Agriculture edited by Charlie M. Shackleton, Margaret W. Pasquini, Axel W. Drescher.
[9] Food Production and Consumption Trends in Sub-Saharan Africa: Prospects for the Transformation of the Agricultural Sector Nicolas Depetris Chauvin, Francis Mulangu and Guido Porto.
[10] Mariga, I. K., Mativha, L. & Maposa, D. (2012). Nutritional assessment of a traditional local vegetable (Brassica oleracea var. acephala). Journal of Medicinal Plants Research, 6, 784–789. (I couldn’t access Foeken and Owuor’s original paper because it was behind a paywall)