You have spent days in the data. You have 40 charts. Your audience has 10 minutes. Data storytelling means turning an analysis into a message that is understood, remembered and acted on.
Exploring is not explaining
- Exploratory analysis is for you: you search, test and make mistakes. You produce a lot of charts.
- Explanatory analysis is for your audience: you convey what matters. You show few, carefully chosen charts.
The reflex to show all the work done (“to prove we worked hard”) drowns the message. Your work shows in the clarity of the conclusion, not in the number of slides.
The three questions before you start
- Who? Who is the audience, what do they already know, what do they expect, who decides?
- What? What should they know or do at the end? A decision, a trade-off, a realization?
- How? Live meeting, document read alone, email, dashboard? A document read without you must be self-contained; an oral presentation can be cleaner.
The key message
The key message (big idea) fits in one complete sentence that contains a point of view, a number and a stake.
- Weak: “Q3 sales analysis.”
- Weak: “Sales evolved in a mixed way.”
- Strong: “The 6% drop in Q3 revenue comes almost entirely from the Southern store channel; reopening on Saturday afternoons would recover about 40,000 euros per quarter.”
The message rests on a precise calculation. For example, to find out which segment explains a decline:
import pandas as pd
ventes = pd.DataFrame({
"segment": ["magasin Nord", "magasin Sud", "web Nord", "web Sud"],
"t2": [120_000, 150_000, 80_000, 60_000],
"t3": [119_000, 124_000, 83_000, 59_400],
})
ventes["ecart"] = ventes["t3"] - ventes["t2"]
total_t2 = ventes["t2"].sum()
variation_totale = ventes["t3"].sum() / total_t2 - 1 # about -6%
ventes["contribution"] = ventes["ecart"] / total_t2 # as a proportion of Q2 revenue
principal = ventes.loc[ventes["ecart"].idxmin()]
part_baisse = principal["ecart"] / ventes["ecart"].sum() # share of the total decline
print(f"{variation_totale:.1%}", principal["segment"], f"{part_baisse:.0%}")
Result: the South store explains more than the entire decline (the other segments offset it slightly). That is the heart of the message.
The pyramid: conclusion first
School reports follow the order “context, method, results, conclusion”. A professional audience wants the opposite:
- The answer: the key message and the recommendation.
- The arguments: two to four points that support it, each with a chart.
- The details: method, data, limits, in an appendix.
If the audience only listens to the first minute, they leave with the essentials.
Adapt to the audience
The same result is told differently to the CEO (decision, financial impact, risk), to the regional manager (their scope, concrete actions) and to the data team (method, robustness). Preparing these versions avoids cramming everything into a single document.
Key takeaways
- You present to help people understand and decide, not to show everything.
- Who, what, how: settle these before the first chart.
- One key-message sentence, with numbers; conclusion first.
Exercises
Solutions and tests stay on the server. In the student space, your code is run and checked, and the tutor gives hints before the solution.
Find the segment that explains the decline
From
ventes, calculatevariation_totale(relative change in total revenue between t2 and t3),segment_principal(the segment whose t3 - t2 gap is the most negative) andpart_baisse(this segment’s gap divided by the total gap).import pandas as pd ventes = pd.DataFrame({ 'segment': ['magasin Nord', 'magasin Sud', 'web Nord', 'web Sud'], 't2': [120_000, 150_000, 80_000, 60_000], 't3': [119_000, 124_000, 83_000, 59_400], }) # variation_totale = ... # segment_principal = ... # part_baisse = ...The best key message
Which slide title is the best key message for an executive committee?
- Third-quarter sales
- Mixed sales trends across channels
- Revenue falls 6% in Q3, entirely because of the Southern stores: we propose reopening them on Saturday afternoons
- Chart 3: revenue by segment
Write the framing of a presentation
Choose a recent analysis (or the one from exercise 1). In 10 lines maximum, write the framing of your presentation: audience, expected decision, format, key message in one sentence, and the three arguments that support it, in pyramid order.
- The audience and the decision-maker are named
- The expected decision or action is explicit
- The format (meeting, document, email, dashboard) is specified
- The key message is a complete sentence with at least one number
- At most three arguments support the message, each tied to a chart or a number
End-of-lesson quiz
What is the main difference between exploratory and explanatory analysis?
- Exploratory uses Python, explanatory uses PowerPoint
- Exploratory is for finding, explanatory is for conveying a chosen message
- There is none
According to the pyramid principle, where do you start?
- The context and the method
- The conclusion or the recommendation
- The limits of the analysis
For a document read without you, you need:
- Very clean slides with no text
- A self-contained document, with action titles and explanations
- Only tables
The rest of the course
- Choosing the Right Chart18 min
- Decluttering and Highlighting18 min
- Structuring the Narrative: Action Titles and Annotations18 min
- Recommendations, Uncertainty and Presentation Format18 min